Toss-up races are dominating the final days of the unpredictable midterm elections, pushing the heated battle for the Senate down to the wire.

How those races shake out will determine which party controls the upper chamber next year, as Democrats appear increasingly confident they'll retake the House.

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President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE and Republican leaders have homed in on expanding the Senate majority, with Trump barnstorming the country for eleventh-hour stops in key states to energize his base in an effort to drive more GOP voters to the polls on Tuesday.

Republicans, who hold a narrow 51-49 majority, have a much stronger chance of hanging on in the run-up to Election Day, when they have only one vulnerable incumbent, Sen. Dean Heller Dean Arthur HellerOn The Trail: Democrats plan to hammer Trump on Social Security, Medicare Lobbying World Democrats spend big to put Senate in play MORE (Nev.), on the ballot.

Even with a promising GOP map, Doug Heye, a strategist and former spokesman for the Republican National Committee, said the battle for the Senate wasn’t likely to feature many “blowouts anyways,” meaning several races may very well be decided in the remaining days.

The overall showdown is being fought against a backdrop of deep polarization, political violence and racial animus that’s left a degree of uncertainty heading into Tuesday.

“It comes from underwhelming candidates in some cases and a polarized population in a lot of states,” Heye said. “In other cases, you have candidates who match up against each other pretty well.”

Democrats face a much narrower, but not impossible, path to winning back the Senate. Historically, the GOP should lose Senate seats during the first midterm of a Republican presidency, but Democrats are playing defense in 10 states that Trump carried in 2016, and where he remains popular.

They'll need to run the table — flipping two GOP seats and holding on to their own vulnerable incumbents — if they want to win the Senate.

Democrats argue that the political atmosphere is trending in their direction as their candidates focus on issues like health care and protections for people with pre-existing conditions, areas where they feel they have an advantage because of GOP plans to repeal ObamaCare.

But they face growing headaches in states like North Dakota, where Sen. Heidi Heitkamp Mary (Heidi) Kathryn HeitkampCentrists, progressives rally around Harris pick for VP 70 former senators propose bipartisan caucus for incumbents Susan Collins set to play pivotal role in impeachment drama MORE (D) is trailing her GOP challenger, Rep. Kevin Cramer Kevin John CramerMomentum growing among Republicans for Supreme Court vote before Election Day On Paycheck Protection Program, streamlined forgiveness is key McConnell shores up GOP support for coronavirus package MORE, by more than 11 percentage points, according to an average of polls by RealClearPolitics.

In a sign that Republicans feel they are poised to pick up the seat, Trump has no plans to visit North Dakota in the waning days of the midterm campaign.

Democrats, however, aren’t counting Heitkamp out. She was down almost 6 points in 2012 before eking out a 1-point victory.

David Bergstein, a spokesman for the Democratic Senatorial Campaign Committee, said Democrats have “cut our potential liabilities” and forced Republicans to spend millions of dollars defending seats in deeply red states.

“That’s a pretty significant change in the underlying dynamics of the map since the start of the cycle,” he said. “We needed to shrink our defensive battleground while putting every realistic target in play — that’s exactly what’s happened.”

If Democrats lose just one of the seats that they’re defending they’ll need to win in three out of their four potential pick up opportunities: Nevada, where Heller is the only GOP senator running in a state that Hillary Clinton Hillary Diane Rodham ClintonButtigieg stands in as Pence for Harris's debate practice Senate GOP sees early Supreme Court vote as political booster shot Poll: 51 percent of voters want to abolish the electoral college MORE won in 2016, as well as seats in Arizona, Tennessee and Texas.

Underscoring the electoral advantage for Republicans, they could lose in Nevada, where Heller is facing off against freshman Rep. Jacky Rosen Jacklyn (Jacky) Sheryl RosenHillicon Valley: Election officials prepare for new Russian interference battle | 'Markeyverse' of online fans helps take down a Kennedy | GOP senators unveil bill to update tech liability protections Google, Apple, eBay to meet virtually with lawmakers for tech group's annual fly-in Senate Democrats demand answers on migrant child trafficking during pandemic MORE (D), and still hold on to the chamber with a 50-50 split, with Vice President Pence available to cast any tie-breaking votes.

Bergstein said Democrats believe they have a “strong path to victory” in Arizona, Nevada and Tennessee. The nonpartisan election handicapper Cook Political Report lists each race as a toss-up.

Rep. Kyrsten Sinema (D-Ariz.) is ahead of Rep. Martha McSally Martha Elizabeth McSallyTumultuous court battle upends fight for Senate Grassley, Ernst pledge to 'evaluate' Trump's Supreme Court nominee The Hill's Campaign Report: Biden goes on offense MORE (R-Ariz.) by less than 1 percentage point in their Senate race, though some recent polls have her ahead by as many as 6 points. A win by Sinema would give Democrats their first Senate victory in Arizona since the mid-1990s.

In Nevada, Heller is up by an average of 2 points, according to RealClearPolitics, though a recent CNN poll showed Rosen up by 3 points. In a potential sign of trouble for Heller, Democrats are leading in the state’s early voting tally by roughly 3.5 percent, which will put pressure on the GOP senator to drive up his margin among independents on Tuesday. And after predicting a Heller victory in 2012, Jon Ralston, a veteran Nevada political observer, said on Sunday he believes Heller will lose this year.

Heller and McSally have both hewed closely to Trump as they try to gin up support from the president’s fervent base. Trump stumped for both candidates last month, but GOP sources told CNN that he was asked to avoid rallies in Arizona and Nevada in the final days of the campaign.

Two of Democrats’ long-shot pickup opportunities — Tennessee and Texas — appear to be fading. Rep. Marsha Blackburn Marsha BlackburnTaylor Swift on National Voter Registration Day: 'We need everyone' Democrats smell blood with new DHS whistleblower complaint Hillicon Valley: Election officials prepare for new Russian interference battle | 'Markeyverse' of online fans helps take down a Kennedy | GOP senators unveil bill to update tech liability protections MORE (Tenn.) and Sen. Ted Cruz Rafael (Ted) Edward CruzTrump argues full Supreme Court needed to settle potential election disputes Press: Notorious RBG vs Notorious GOP The Hill's Morning Report - Sponsored by Facebook - Washington on edge amid SCOTUS vacancy MORE (Texas) both hold firm, single-digit leads in their respective races.

Republicans are hoping close contests in red states, where Democrats are trying to emerge victorious in states carried by Trump in 2016, will allow them to increase their majority.

A GOP strategist said Republicans have a “great map” but are battling history that shows the party in power doesn’t get “a favorable outcome” two years into a Republican administration.

“I don't think anyone is surprised in this environment that we're at a place where there are so many toss-ups,” the strategist added.

In addition to North Dakota, Democratic incumbents in Florida, Indiana and Missouri are locked in tight races viewed by strategists and election handicappers alike as toss-ups. Strategists from both parties say Missouri, Indiana and North Dakota are the three closest contests where Democrats are on the ballot.

Meanwhile, Montana has appeared to come back into range for Republicans, with millions of dollars in advertising flooding the state and the White House’s renewed focus on knocking off Sen. Jon Tester Jonathan (Jon) TesterPence seeks to boost Daines in critical Montana Senate race This World Suicide Prevention Day, let's recommit to protecting the lives of our veterans Filibuster fight looms if Democrats retake Senate MORE (D), who has maintained a narrow but steady lead in the polls.

Trump is crisscrossing the country in the final days of the election cycle with the hope of boosting turnout among his fervent base, even though his name is not atop the ticket. He’s making repeat trips to Missouri, Indiana, Montana, Florida and West Virginia, while steering clear of Democrats’ two best pick-up opportunities: Nevada and Arizona.

Katie Martin, a spokeswoman for the National Republican Senatorial Committee, said Republicans believe GOP candidates will benefit from Trump's presence.

"A visit from President Trump is one of the biggest boosts for Republican Senate campaigns, and we consistently see a bump in polling after rallies," she said.

The president has largely focused his closing message on immigration, a red-meat issue for his base. But some Republicans worry that his call to end birthright citizenship through an executive order a week before the midterms puts the GOP in an uncomfortable spot.

Trump also prompted fierce backlash from Democrats — and outspoken critic Sen. Jeff Flake Jeffrey (Jeff) Lane FlakeJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Republican former Michigan governor says he's voting for Biden Maybe they just don't like cowboys: The president is successful, some just don't like his style MORE (R-Ariz.) — over a new campaign ad about how Democrats would handle immigration, focusing the spot on a deported Mexican man who killed two sheriff’s deputies after reentering the U.S.

Republicans hope that rhetoric won’t overshadow the party’s overall message on immigration, and its emphasis on economic growth over the past two years.

“Immigration drives our base and drives it more than it does for Democrats,” said Heye. “It can be a base motivator, but there are other ways to do that than putting your own party at odds with itself and on defense.”