The race for the Senate has become an all-out brawl as Republicans fiercely defend their slim 51-49 majority against a Democratic Party sensing momentum is on its side.

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 (R-Ky.) acknowledged Tuesday that Republicans will be facing a “storm” in the November midterm elections, describing the races as "a knife fight in an alley," while adding he was still hoping that Republicans would retain their majority "when the smoke clears."

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Republicans still have the upper hand, as Democrats must defend 10 seats carried by 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 2016, but Democrats believe they have a narrow path to take the Senate.

Here are the top 10 seats most likely to flip.

1. 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.)

The freshman senator is one of five Democrats up for reelection in a state that Trump won by double-digits, with North Dakota giving the president a 36 point margin over Democratic presidential nominee 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 in 2016.

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.), the state’s lone House member since 2013, has been a loyal ally of Trump, who persuaded him to run after he initially declined. The last public poll from June had Cramer up 4 points, and Republicans say private polls show him ahead.

But Democrats contend that GOP polling was off in Heitkamp’s 2012 race, which showed her down consistently. She ultimately won by less than a point.

The escalating trade war has become a central issue in the race. And like many Democrats, Heitkamp is hoping her opposition to an anti-ObamaCare lawsuit will be a successful wedge issue.

Meanwhile, Cramer is hitting Heitkamp for opposing the tax cuts passed by Congress last year, Trump’s most significant legislative achievement to date.

But Heitkamp has a strong personal brand, and she believes she can win over enough moderate voters. Ultimately, political observers say the race could hinge on Trump, and whether voters prefer a consistent Trump ally or a senator with an independent streak.

2. 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 (R-Nev.)

Heller remains the most vulnerable GOP senator as the only one up for reelection in a state that Clinton won. Nevada is a swing state turning bluer with demographic changes and elevated Democratic enthusiasm.

The tax law has taken center stage as Heller has touted his role in crafting the law that 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) opposed in the House.

But Rosen has campaigned saying the tax cuts largely benefit the wealthy and corporations, and has knocked Heller’s support for repealing ObamaCare.

Heller has defended himself by saying that the GOP-led health-care bill, which ultimately failed, wouldn’t have touched Nevada’s Medicaid expansion, and said in his own ad that he’s “fighting to protect pre-existing conditions.”

Polls consistently show a tight race, with the latest survey from Suffolk University/Reno Gazette Journal finding another dead-heat race with Rosen nominally ahead.

3. 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.)

McCaskill has always faced a tough path to a third term. Trump won Missouri by nearly 20 points, and Republicans are hoping to repeat that in the race between her and state Attorney General Josh Hawley.

Hawley is considered one of the GOP’s top recruits in 2018, and he has made support for Trump and Supreme Court nominee Brett Kavanaugh centerpieces of his challenge to McCaskill.

The two-term Democrat, on the other hand, is banking on liberal dissatisfaction with the president and Republican efforts to chip away at the Affordable Care Act to hand her a win in November.

Working in McCaskill’s favor is her reputation as an adept campaigner and fundraiser. She’s far ahead of Hawley in the money race, having brought in more than $20 million this cycle compared to her challenger’s $4.75 million.

4. 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 (D-Ind.)

Donnelly has maintained a more comfortable lead in the polls despite being considered one of the most vulnerable incumbents. The latest poll from NBC News has him up 6 points.

But Donnelly is still facing an uphill battle against Republican businessman Mike Braun in a state that Trump won by 19 points.

Both candidates have liabilities: Democrats have criticized Braun over his business record at his trucking and auto parts distribution companies, while Braun and allies have hit Donnelly with outsourcing attacks related to a family company.

Braun is touting his relationship with Trump, who’s made several visits to Indiana for rallies and has made Donnelly a top target.

5. 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 (D-Fla.)

After a low-key, uncontested primary season, Nelson is facing formidable opponent in the form of Florida’s wealthy Republican governor, Rick Scott. And while the three-term Democrat has easily won reelection in the past, the challenge from Scott has turned the race into a nail-biter.

Scott faced only a nominal primary challenge, but saturated Florida’s expensive airwaves early with spots attacking Nelson — a strategy that allowed him to set the terms of the race early.

Nelson is on the air now, but only after weathering months of negative ads and headlines describing his reelection bid as lackluster.

But Democrats argue that there’s a silver lining for Nelson. Scott has drastically outspent him but has yet to surge ahead in the polls. The two-term governor also has a history of winning by razor-thin margins in Republican wave years, and Democrats say that, this year, they are the ones with the momentum.

6. Arizona’s open seat

Democrats have felt bullish about Arizona for months with Rep. Kyrsten Sinema (D-Ariz.) running for the seat being vacated by GOP 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.

Sinema’s a prolific fundraiser and received little on-air pushback while a contentious GOP primary raged on, which ultimately ended up nominating GOP establishment candidate 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.).

During the primary, Sinema polled consistently ahead of McSally. But with the late August primary in the rearview mirror, new polls show McSally quickly consolidating support — and slightly leading Sinema.

Sinema, a Blue Dog Democrat, is running as a moderate in a state that Trump won by less than 5 points, but one that also hasn’t sent a Democrat to the Senate in three decades.

McSally, the first female combat pilot in the U.S., is leaning heavily on her military credentials and touting border security.

McSally is looking to turn up the heat six weeks out from the early voting kickoff. Her latest ad uses footage of Sinema in a pink tutu, and goes on to note that McSally served in the military while her opponent protested the Iraq War and “denigrated” the troops. Politifact, however, has labeled those claims “mostly false,” saying it had found no evidence that Sinema had disparaged American troops.

7. 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 (D-W.Va.)

On paper, Manchin should be an easy target for Republicans in their bid to expand their Senate majority; West Virginia gave Trump one of his largest margins of victory in 2016.

But he has managed to hold a lead over his GOP opponent, state Attorney General Patrick Morrisey, who weathered a brutal primary season earlier this year.

Still, Trump remains highly popular in West Virginia and has hit Manchin during multiple rallies in the state over his opposition to the GOP-led tax overhaul.

Manchin has said he voted against the measure because it effectively killed the requirement that people purchase health insurance.

At the same time, however, Manchin has found himself on the receiving end of an attack by the National Rifle Association, which endorsed Morrisey and debuted an ad this month targeting Manchin for his stance on gun control.

8. 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.)

Trump may have won Montana by a hefty margin in 2016, but Tester is betting that his support for measures, including a pair of veterans’ health-care bills, will convince voters to send him back to Washington for a third term in the Senate.

He’s facing a general election challenge from Republican Matt Rosendale, the state auditor who has made his support for Trump a key focus of his campaign. In response, Tester has played up his willingness to work with the president on veterans’ issues. One in 10 Montanans are veterans.

Still, Tester and Gov. Steve Bullock are the last remaining Democrats serving in statewide offices in Montana and Trump has hit the two-term senator repeatedly for raising concerns about his former pick to lead the Veterans Affairs Department, Ronny Jackson.

9. Tennessee’s open seat

Tennessee makes its debut on the list, with polls continuing to show a close race between former Gov. Phil Bredesen (D) and 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 (R-Tenn.) for the seat being vacated by GOP Sen. Bob Corker Robert (Bob) Phillips CorkerHas Congress captured Russia policy? Tennessee primary battle turns nasty for Republicans Cheney clashes with Trump MORE.

After being out of office since 2011, Bredesen is looking to convince voters that he won’t be a rubber stamp on the Democratic Party’s agenda as he campaigns in a state that went for Trump by 26 points. And he hasn’t said whether he’ll support Sen. Charles Schumer Chuck SchumerSenate Democrats introduce legislation to probe politicization of pandemic response Schumer interrupted during live briefing by heckler: 'Stop lying to the people' Jacobin editor: Primarying Schumer would force him to fight Trump's SCOTUS nominee MORE (D-N.Y.) for leader.

Bredesen has proven before that he can sway Republicans, but will have to do so in an increasingly polarized environment. Meanwhile, Blackburn, who’s built up a conservative portfolio over the years, is running a more nationalized campaign, touting her support for Trump.

10. Sen. Tammy Baldwin Tammy Suzanne BaldwinKeep teachers in the classroom Cher raised million for Biden campaign at LGBTQ-themed fundraiser Democrats seek balance in backing protests, condemning violence MORE (D-Wis.)

Trump’s narrow win in Wisconsin in 2016 fueled hope among Republicans that they could pick up a second Senate seat in the Badger State. But Baldwin was never among those mentioned by Democratic and Republican leaders as being particularly vulnerable.

She is now. Baldwin is the most liberal of the Senate Democrats up for reelection this year in states Trump won, and her Republican opponent, state Sen. Leah Vukmir, has been bullish in casting the one-term incumbent as too far to the left.

Overall, the race still looks good for Baldwin. Trump’s approval rating is underwater in Wisconsin at 36 percent, according to an NBC News/Marist poll released in July, and Baldwin is outpacing Vukmir in fundraising.

Still, Republicans say that there’s time for the race to pick up and Democrats acknowledge that it’s still competitive.

-- This story has been updated at 5:35 p.m.