Senate Republicans believe they are on the cusp of increasing their majority in the midterm elections despite historic headwinds that seem likely to cost the GOP control of the House.

Republicans believe they have growing momentum to pick up seats in North Dakota, Missouri and Indiana, while races in Montana and Florida remain tight.

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The GOP is feeling more bullish about not losing any of their own incumbents up for reelection, a significant turnaround from earlier in the cycle when Nevada 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 was seen as possibly the most endangered Senate incumbent standing.

Surprises are still possible on Tuesday in a quickly changing environment.

Here's a look at the Senate seats more likely to flip:

North Dakota

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-N.D.) is viewed as the most vulnerable Democratic incumbent in the final weeks before the midterm elections, as she’s trailed behind 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 (R-N.D.) in the polls.

Democrats aren’t counting Heitkamp out after she squeaked out a victory in 2012. They argue that Heitkamp still has a path if she can hit her targets in the eastern part of the state, win over moderates and turn out Native American voters.

The election will come less than a week after tribes unsuccessfully challenged a voter ID law that requires a current address for the identification to be valid. Native Americans strongly backed Heitkamp in 2012.

Though recent polls have shown a slight tightening in the race, Heitkamp is behind in the polls by an average of more than 11 percentage points, according to RealClearPolitics.

The race is being handicapped by election watchers as leaning toward Republicans. And 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, in a sign that Republicans are feeling good about the state, isn’t expected to make a campaign stop in the final days before the election.

Missouri

Sen. Claire McCaskill Claire Conner McCaskillMomentum growing among Republicans for Supreme Court vote before Election Day Democratic-linked group runs ads in Kansas GOP Senate primary Trump mocked for low attendance at rally MORE (D-Mo.) has been a top target since the start of the cycle, with Republicans convinced they would have beaten her in 2012 if her GOP opponent, then-Rep. Todd Akin (Mo.), hadn’t sparked a political firestorm by saying that female bodies have a way of preventing pregnancy from “legitimate rape.”

But she’s managed to keep her race close despite Trump winning her state by roughly 15 percentage points in 2016.

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Several polls in the final month before the midterms, including a Fox News poll released last week, have shown the race between McCaskill and Attorney General Josh Hawley (R) locked in a tie. Others have Hawley with a slight lead but well within the margin of error.

McCaskill has tried to align herself with Trump and distance herself from the more progressive elements in her party, telling Fox News that she isn’t one of those “crazy Democrats” and that she “100 percent” supports Trump blocking a migrant caravan from entering the country.

But her strategy earned her no reprieve from Trump, who said during a rally in Missouri days before the midterms that the voters would “retire far-left Democrat Claire McCaskill.”

Indiana

Democratic Sen. Joe Donnelly Joseph (Joe) Simon DonnellyTrump meets with potential Supreme Court pick Amy Coney Barrett at White House Names to watch as Trump picks Ginsburg replacement on Supreme Court Momentum growing among Republicans for Supreme Court vote before Election Day MORE (Ind.) is trying to hold on to his seat in a state Trump won by roughly 16 points in 2016 and where he is the lone Democrat to hold a statewide office.

Donnelly has tried to tie himself to Trump, including becoming the first Democratic senator to open the door to considering Trump’s controversial effort to change who qualifies for birthright citizenship.

A Fox News Poll showed Donnelly with a 7-point lead over businessman Mike Braun. But the race remains rated a toss-up, with Republicans viewing it, Missouri and North Dakota as their best chances to pick up seats currently held by Democrats.

Arizona

Arizona is emerging as Democrats’ best shot to flip a Republican-held seat in an otherwise brutal Senate midterm map.

Republicans hoped that when 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.), a frequent critic of the president, announced his retirement last year it would give the party a better shot at protecting the Senate seat.

But while Republicans got the candidate they wanted when 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.) managed to survive a brutal primary election, she’s remained locked in a tight battle with Rep. Kyrsten Sinema (D-Ariz.).

Unlike in Nevada, where Democrats are trying to unseat an incumbent, the open race appears to be giving Sinema and her party an easier, but still heavy, lift.

Sinema is leading in the race on average by a percentage point, according to RealClearPolitics. And a Fox News poll released last week had the race locked in a tie.

Nevada

Heller has been a top target since the start of the 2018 cycle as the only Senate Republican running for reelection in a state won by 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.

But he’s managed to stay in his race against 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-Nev.) despite being targeted by more than $33.5 million in negative spending from outside groups.

Heller has stuck closely to Trump as he’s searched for a path to victory in the battleground state, and RealClearPolitics shows he’s leading by 2 percentage points on average.

But, 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.

Montana

How close the race in Montana is during the waning days of the 2018 election cycle depends on which party you ask.

Democrats, while acknowledging the race is close, are quick to note 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-Mont.) has maintained a steady lead in the polls for weeks despite months of battering by Trump and Donald Trump Jr. Don John Trump'Tiger King' star Joe Exotic requests pardon from Trump: 'Be my hero please' Zaid Jilani discusses Trump's move to cancel racial sensitivity training at federal agencies Trump International Hotel in Vancouver closes permanently MORE

Tester, according to RealClearPolitics, has a more than 4-point lead on average. But other recent polls show the race being as narrow as 2 or 3 percentage points, putting it within the margin of error.

Republicans are more bullish, believing the Tester-Trump feud has moved the race their way. And Republicans are hoping that a last-minute visit by Trump, who remains deeply popular in the state, will help vault Matt Rosendale into the lead just in time for Tuesday.

Florida

Democrats are feeling bullish about Sen. Bill Nelson Clarence (Bill) William NelsonDemocrats sound alarm on possible election chaos Trump, facing trouble in Florida, goes all in NASA names DC headquarters after agency's first Black female engineer Mary W. Jackson MORE’s chances in Florida despite Gov. Rick Scott having gone up with TV ads months before Nelson and pouring more than $63 million into his race to defeat the incumbent.

The Democratic Senatorial Campaign Committee on Friday pointed to early Republican optimism about the race as an example of a “GOP talking point busts,” noting that Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellPelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Senate GOP aims to confirm Trump court pick by Oct. 29: report Trump argues full Supreme Court needed to settle potential election disputes MORE’s (R-Ky.) prediction that Democrats would leave Nelson “for dead” by mid-September didn’t pan out.

Democrats point to two main factors as the reason for their optimism: The composition of Florida, which has the reputation as a swing state, and the excitement over Tallahassee Mayor Andrew Gillum, the Democratic gubernatorial nominee.

Nelson only has a narrow 2-point lead in the race according to several polls, but FiveThirtyEight gives him a 2-in-3 chance of keeping the seat.

West Virginia

Sen. Joe Manchin Joseph (Joe) ManchinThe Hill's Morning Report - Sponsored by Facebook - Trump, GOP allies prepare for SCOTUS nomination this week Trump meets with potential Supreme Court pick Amy Coney Barrett at White House Names to watch as Trump picks Ginsburg replacement on Supreme Court MORE was initially viewed as one of the most vulnerable Democratic incumbents up for reelection in a state won by Trump by roughly 42 points — his biggest margin of victory in 2016.

But while many of his red-state colleagues have seesawed in and out of front-runner status in their respective races, Manchin has maintained a solid lead over Attorney General Patrick Morrisey in public polling in the state over for weeks.

Strategists in both parties attribute his advantage to his gifts as a retail politician and name ID in a state where he served as governor before joining the Senate.

Republicans are hoping a final visit by Trump to the state could provide Morrisey with momentum heading into election day.

But The Washington Post reported that the Senate Leadership Fund, a super PAC aligned with McConnell, is no longer airing television advertisements in the Mountain State.