Democrats are expected to take control of Congress in a "blue wave" Tuesday, but for that to happen a handful of key House and Senate races have to break their way.

As voters head to the polls, there are at 10 key House and Senate races to watch that will tell whether or not a "blue wave" is in store for Americans.

The House and Senate Makeup

Democrats need to pick up 23 seats in order to take control of the House, which most major polling houses peg as a "more than likely" scenario. The current House electoral map has 202 seats locked-in for Democrats and 194 for Republicans, which leaves 39 races, considered "toss ups," for either party to snag on Election Day.

Even with a large number of toss-up races, Democrats appear to have a sizable lead over Republicans.

Sabato's Crystal Ball predicted Monday Democrats will pick up in excess of 30 seats Tuesday. RealClearPolitics expects Democrats to take roughly 26.5 seats.

Republican strategists have told the Washington Examiner in recent weeks they believe keeping the House red is feasible, pointing out that many of the races are within the margin of error. They argue that with some luck and last-minute campaigning, Republicans could see enough of a push to maintain their majority. Despite some optimistic predictions, the conventional wisdom has the House going for Democrats.

In the Senate, Democrats need to pick up three seats in order to take the majority. Republicans currently hold a slim 51-49 majority in the upper chamber and look poised to not only maintain control of the upper chamber, but add additional seats.

FiveThirtyEight has Republicans' chances of keeping the Senate at just over 83 percent Monday.

House Races To Watch

1. Kentucky 6th — Rep. Andy Barr, R-Ky., vs. Democrat Amy McGrath

Barr and McGrath are locked in a neck and neck race heading into Tuesday's vote. Barr led McGrath by just 1 percentage point in September, and his narrow lead has disappeared in recent weeks. The candidates are currently locked in a "tie," according to a New York Times/Sienna poll published Sunday.

The race is one to watch for many reasons, particularly because the district is so demographically unique that it could shed some light on the possibility of a "blue wave."

Kentucky 6th has more registered Democratic voters than Republican voters. The district and the region has an entrenched Democratic background, and Sen. Rand Paul, R-Ky., lost there in 2016. President Trump won Kentucky's 6th district by 15 percentage points in 2016, and 2012 Republican presidential nominee Mitt Romney took it in 2012.

Trump's success in the district, along with his presence on the campaign trail during the midterms, has made him a key figure in the race.

Barr is positioning himself as a key Trump ally. Unlike many Republicans in tight races nationwide, Barr chose to campaign on-stage with the president this cycle. Like Trump, Barr won the district decidedly in 2016, taking 22 percent of the vote. His landslide victory in 2016 makes the tight race this cycle interesting in light of the "blue wave" predictions.

McGrath is using Barr's voting Republican voting record and failure to criticize the president's controversial statements as evidence that he is "as partisan as it gets." McGrath is running an anti-establishment candidate who is running for Congress to help tone down partisanship in Washington.

2. Virginia 10th — Rep. Barbara Comstock, R-Va., vs. Democrat Jennifer Wexton

Comstock is an example of the conundrum Trump presents for Republican candidates this cycle. While Comstock was well-liked within the district, Trump is not. The president's low favorability, coupled with Comstock frequently voting with the administration, makes this race one of the Democrats' best chances for flipping a House seat.

Comstock, whose district went for Clinton by 10 percentage points, has spent much her time and political capital avoiding attacks from her opponent, Wexton, that she is a pro-Trump Republican that votes with his agenda 97.8 percent of the time. Wexton is currently running attack ads against Comstock smearing the congresswoman as "Barbara Trump-stock."

The two-term incumbent is trying to signal she isn't a completely Trump-aligned candidate, pointing to her vote against repealing and replacing Obamacare in 2017.

"I'm my own woman, and I focus on the priorities of my constituents," Comstock told CNN in October. "I have worked with a Republican governor and a Democrat governor."

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Despite her efforts to convince voters otherwise, Comstock found herself down roughly 8 against Wexton on the eve of Election Day. Political strategists look at races like Comstock's as proof that voters are astute and keenly aware of when candidates are trying to " pull the wool over their eyes."

3. California 48th — Rep. Dana Rohrabacher, R-Calif., vs. Democrat Harley Rouda

California's 48th Congressional District has long been considered a Republican stronghold. Republicans have a 10-point advantage in voter registration. Still, Hillary Clinton took the district by 2 percentage points in 2016.

Rouda holds a slim 1 percentage point lead over Rohrabacher, according to a New York Times/Sienna poll released Sunday. The race has jockeyed back and forth, with either candidate locking a small lead over the other since July. For much of September, the race was considered a "tie."

Voter turnout estimates have the race going for Rouda by 2 percentage points.

Rohrabacker voted to repeal and replace Obamacare, but, notably, voted against the 2017 GOP tax reform bill. He is also known as one of the most pro-Russia members of Congress, which could hurt him in an election year where Democrats are focused on special counsel Robert Mueller's investigation into Russian meddling in the 2016 presidential election.

Rouda, once a Republican, has moved to the Democratic Party because he said that Republicans under Trump have left their constituents behind. The Democratic candidate was one of the few to get both the backing of progressive groups and the Democratic Congressional Campaign Committee.

4. Iowa First — Rep. Rod Blum, R-Iowa, vs. Democrat Abby Finkenauer

Iowa's 1st Congressional District went for Barack Obama in 2008 and 2012 by double-digit margins. In 2016, Trump took the district over Clinton by 4 percentage points. The president was the first Republican presidential candidate to carry Iowa's Dubuque County since Dwight D. Eisenhower in 1952.

Given the swift change in presidential voting records, Iowa's 1st has many eyes on it as voters watch for an impending "blue wave." The district has been called the most important swing district in 2018 and Blum is considered one of a few Republican members to "likely" lose Tuesday.

RealClearPolitics polling average has Finkenauer up 9.5 percentage points, and other outlets have the Democrat with a sizable lead over Blum.

The race is setting up to be an election that could show party strategists how they will approach the 2020 election cycle.

Voters will choose between a pro-Trump conservative House Freedom Caucus member Blum, or a young, progressive, pro-labor candidate with Finkenauer.

5. Florida 26th — Rep. Carlos Curbelo, R-Fla., vs. Democrat Debbie Mucarsel-Powell

Florida's 26th is a heavily Hispanic district that went for Obama in 2012 and Clinton in 2016 by double-digit percentages.

Cook Political Report and FiveThirtyEight have the race labeled a "tossup," but other polling houses have it leaning slightly Republican.

Incumbent Curbelo is having a difficult time against Mucarsel-Powell, who immigrated to the U.S. from Ecuador as a teenager. Mucarsel-Powell has said the impetus for her congressional bid was Curbelo's vote to repeal and replace Obamacare, a vote that Curbelo has tried to distance himself from with his stances on immigration and numerous battles with the president. Curbelo also voted for the 2017 tax reform bill.

Curbelo has called Trump's rhetoric reprehensible and claimed it is an illustration of the difficult political climate America finds itself in this cycle. Despite trying to paint himself as a Republican that can work with Trump and call him out when necessary, voters don't appear to be buying the message.

Mucarsel-Powell held a slim 1-percentage point lead over Curbelo in October, but Curbelo has managed to edge out a small 3-percentage point lead in the final days before the election.

Senate Races To Watch

1. Indiana — Sen. Joe Donnelly, D-Ind., vs. Republican Mike Braun

Once one of the most Democratic states in the Great Lakes region, Indiana is now considered a Republican stronghold. Indiana voted overwhelmingly for Trump in 2016 and is the home state of Vice President Mike Pence.

Prior to Democratic Sen. Joe Donnelly taking the seat, the state has elected a Republican senator for over three decades. Braun, like many GOP Senate candidates, trails his opponent within the margin of error. RealClearPolitics' polling average has incumbent Donnelly up roughly 0.4 percentage points heading into Election Day.

One factor many strategists are looking at in the race is what pundits are calling the "Kavanaugh effect."

In such a heavily Republican state, Donnelly could have done himself some damage when he voted against Supreme Court Justice Brett Kavanaugh's nomination.

The vote doesn't appear to have hurt the senator as much as some expected it might.

FiveThirtyEight has Democrats' chances of winning the race at just under 71 percent, Sabato's Crystal Ball has the state leaning Republican.

2. Arizona — Open seat, Rep. Martha McSally, R-Ariz., vs. Rep. Kyrsten Sinema, D-Ariz.

Arizona is considered a Republican state, but the population is slowly moving toward the Democratic Party in recent years. Republicans have maintained a lock on the state legislature for decades and the state has most recently elected two Republican senators — the late Sen. John McCain and retiring Sen. Jeff Flake.

The current Arizona Senate race features Rep. Martha McSally, R-Ariz., and Democratic challenger Rep. Kyrsten Sinema. Like many tight races this year, the Arizona Senate election is well within the margin of error. RealClearPolitics has Sinema over McSally in its polling average by only 0.1 percentage points.

The majority of polls throughout the cycle have pegged Sinema ahead of McSally, but the recent shift has raised some eyebrows.

Similar to the Indiana race, strategists believe McSally could be receiving some boost from Kavanaugh's successful confirmation and the president's rhetoric about the caravan.

Sinema also hasn't done herself any favors with comments that Arizona is a "meth lab of democracy."

3. Nevada — Sen. Dean Heller, R-Nev., vs. Democrat Jacky Rosen

The race features incumbent Sen. Dean Heller, R-Nev., against the formidable Jacky Rosen, who Trump has nicknamed "Whacky Jacky." Heller is the only Republican senator up for re-election in a state that Clinton won in 2016. The state also went for Obama in 2012. Heller only won in 2012 by 1 percentage point.

Heller has spent much of the past two years walking the line between a Trump Republican and a voice in the Senate that can work across the aisle. Rosen has spent the campaign trying to paint Heller as a Republican who supports Trump when it is convenient, but doesn't stick to a solidified ideology.

Because of Heller's narrow 2012 victory and the state going for the Democratic presidential nominee in the past two presidential election cycles, Democrats believe that Nevada is one of their best chances to flip a Senate seat this cycle. They could be correct. RealClearPolitics' polling average has Rosen with a narrow 1 point lead over Heller as of Sunday.

The race is bringing in some serious campaign funds, too. Heller and Rosen have raised nearly $20 million in campaign funds combined. After Arizona and Nevada, Tennessee is the Democrats' next top target among GOP-held Senate seats.

4. Montana — Sen. Jon Tester, D-Mont., vs. Republican Matt Rosendale

Montana typically goes for Republicans in presidential election cycles, but has a serious Democratic streak in local and statewide elections.

Montanans will choose Tuesday between incumbent Sen. Jon Tester, D-Mont., and state Auditor Matt Rosendale. Tester narrowly won his first Senate race in 2006 against then-Sen. Conrad Burns, R-Mont., who is the only Republican to win back-to-back Senate elections in the state's history. In 2012, Tester defeated GOP Rep. Dennis Rehberg to win re-election.

Trump stumped for Rosendale and GOP candidates in Montana last weekend just days before voters head to the ballot box. Republicans control every statewide seat except for Tester's and the governor's mansion.

Tester holds upwards of a 5 percentage point lead over Rosendale heading into Tuesday's election, but the last-minute endorsement of Rosendale from the Libertarian candidate last week caused many to wonder if Tester's lead would diminish in the final days of the election.

Montana's Libertarian candidate dropped out of the state's Senate race in the middle of last week and threw his support behind Rosendale, a move that caused many to wonder if Tester was in trouble.

Libertarian Party nominee Rick Breckenridge told reporters that he was upset about an outside attack ad against Rosendale and urged his supporters to cast their ballots for the Republican.

Montana has a one of the highest libertarian populations in the U.S., and the party's members are active in state and national elections. The group proved difficult for Tester to sway in his prior two elections. The senator's margin of victory in 2006 and 2012 was slimmer than the share of the vote the Libertarian Party candidate received. In 2006, Tester won his first election by 1 percentage point. That same year, the Libertarian candidate took 2.5 percent of the vote. In 2012, the Libertarian candidate took 4.5 percent of the vote when Tester won by a margin of 4 percentage points.

One factor that could prevent Breckenridge's endorsement from having a decided effect, however, is that some Montanans have already cast their votes with Breckenridge's name on the ballot.

5. Missouri — Sen. Claire McCaskill, D-Mo., vs. Republican Josh Hawley

The Missouri Senate race could be the most interesting race this cycle, featuring a well-known Democratic incumbent that is fighting for her political life against an up-and-coming Republican challenger who currently holds a narrow lead in the polls.

Two-term incumbent Sen. Claire McCaskill is facing off against Republican state Attorney General Josh Hawley. Neither candidate has secured more than a 4 percentage point lead throughout the race, but Hawley currently holds a 0.6 percentage point advantage as of Sunday evening.

Hawley has positioned himself as an ally of Trump, which isn't a bad play, given that Trump took the state by 18 points in 2016.

McCaskill, a relatively centrist Democrat, has walked a tight line between occasionally supporting the president's agenda and going with her Democratic colleagues when they decry the president's rhetoric.

Obama lost the state in 2008, which underscores the Republican shift Missouri has undergone for the last decade.

How well Democrats fare in these races will be the true test of whether we are witnessing a blue wave Tuesday night.

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