The Cook Political Report, a nonpartisan election handicapper, shifted three Senate races in favor of Republicans as well as one Senate race toward Democrats less than five weeks out from the midterm elections.

Cook moved Senate races in Montana, Nebraska and New Jersey all in favor of Republicans. 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.) faces one of the most competitive race this cycle, which was shifted from "lean Democratic" to "toss-up." Tester is running against state auditor Matt Rosendale (R) in a state where 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 won by double-digits in 2016.

Jennifer Duffy, a Senate race analyst at Cook Political Report, said Tester initially looked likely to sail to reelection with a comfortable lead in polling and Rosendale facing a crowded primary. Duffy noted that the fight over Supreme Court nominee Brett Kavanaugh's nomination is energizing the GOP base, though she still gives Tester a slight advantage in the toss-up race.

Tester has said he won't vote to confirm Kavanaugh, who faces sexual assault allegations that have threatened to derail his nomination. Kavanaugh has vehemently denied all allegations.

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“The ongoing saga over Brett Kavanaugh’s Supreme Court nomination is also galvanizing the base as Republicans remind voters that Tester voted against Neil Gorsuch, Trump’s first nominee, and has already stated that he will vote against Kavanaugh,” Duffy said.

“While the race is moving to Toss Up based on both public and private polling showing the contest within the margin of error, we’ll put a thumb on the scale for Tester.”

Cook also moved Sen. Bob Menendez Robert (Bob) MenendezKasie Hunt to host lead-in show for MSNBC's 'Morning Joe' Senators ask for removal of tariffs on EU food, wine, spirits: report VOA visa decision could hobble Venezuela coverage MORE’s (D-N.J.) race from "likely Democratic" to "lean Democratic" and Sen. Deb Fischer Debra (Deb) Strobel FischerHillicon 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 Congress botched the CFPB's leadership — here's how to fix it MORE’s (R-Neb.) race from "likely Republican" to "solid Republican."

New Jersey has been a surprisingly competitive race in a blue state where Trump remains unpopular. Menendez faces a tough race from former pharmaceutical CEO Bob Hugin, who’s poured in tens of millions of dollars into his Senate campaign.

Polls have been all over the place with Menendez ahead of Hugin by anywhere from 2 to 11 points. Menendez is still favored to win a third term, but must overcome low favorability that stems from his past corruption case. His trial ended in a hung jury and federal prosecutors later dropped the charges.

“This race is more competitive than a Senate contest in a good year for Democrats in a state as blue as New Jersey should be,” Duffy said. “This is entirely on Menendez and the baggage he brings into the race. He maintains the advantage going into the final stretch, but the race is worth watching and moves to the Lean Democratic column.”

The election handicapper also shifted Ohio’s Senate race toward Democrats. Sen. Sherrod Brown Sherrod Campbell BrownSenate Democrats release report alleging Trump admin undermined fair housing policies Bipartisan praise pours in after Ginsburg's death Emboldened Democrats haggle over 2021 agenda MORE (D-Ohio) has consistently lead Rep. Jim Renacci James (Jim) B. RenacciOhio is suddenly a 2020 battleground Democrats fear Ohio slipping further away in 2020 Medicare for All won't deliver what Democrats promise MORE (R-Ohio) by double-digit margins, despite Trump winning the state by 8 points in 2016. Cook moves the race from "lean Democratic" to "likely Democratic."

Republicans are looking to hold onto their slim 51-49 seat majority in the Senate. The party faces a favorable map, with 10 Democratic senators up for reelection in states Trump carried.

Still, some states like Ohio have already fallen off the map for Republicans. And other competitive races that’ll likely determine which party controls the Senate are still in a dead heat.