The Cook Political Report made a notable rating adjustment in the Montana Senate race, shifting favor to Republicans in the battleground race for the Democratic seat.

The "independent, non-partisan" analyzer, which measures the competitiveness of races nationwide, shifted the Democrat-held Montana Senate seat Thursday from "Lean Democrat" to "Toss Up," a major move in the 2024 election as Democrats cling onto their slim chamber majority.

Sen. Jon Tester, a Democrat, is running for a third term in Montana next fall but faces tough opposition from Republican Tim Sheehy, who is running a high-profile race to flip the blue seat to the Republican Party.

Jessica Taylor of the Cook Political Report wrote that Tester faces "the toughest uphill climb of any incumbent Democrat."

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Republican Montana Senate candidate and former Navy SEAL Tim Sheehy. (Tim Sheehy For Montana)

"Three-term Sen. Jon Tester may well be Democrats’ strongest incumbent this cycle — he still enjoys an approval rating around 60 percent — but the 67-year-old has the toughest uphill climb of any incumbent Democrat, at least purely by the numbers," Taylor said in her analysis.

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Tester's team said the senator is "no stranger to tough fights," pointing to their campaign website in a statement to Fox News Digital.

"Jon's no stranger to tough fights, and this election will be no different," Shelbi Dantic, Montanans for Tester campaign manager, told Fox on the recent rating.

Sheehy's team also released a statement, suggesting that the shift toward the GOP "highlights that Montanans want to elect a political outsider who can bring a new generation of conservative leadership."

Democratic Montana Sen. Jon Tester

Sen. Jon Tester, the only Democrat to hold a statewide seat in Montana, is seeking a third term in the Senate next cycle. (Win McNamee)

"Tim Sheehy's lead in the potential primary matchup highlights that Montanans want to elect a political outsider who can bring a new generation of conservative leadership and finally defeat Jon Tester next year," Sheehy's campaign wrote in a press release after the Cook rating dropped. "It's clear that Tim's message on the airwaves showcasing his military leadership as a former Navy SEAL and businessman and aerial firefighter is resonating with Republican primary voters in Montana."

Democrats will be defending 23 of the 34 Senate seats next cycle, and Montana is set to play a key role in determining control of the Senate alongside West Virginia, Ohio and Arizona. 

Along with Tester, the Cook Political Report also considers seats held by independent Sen. Kirsten Sinema in Arizona and Democrat Sen. Sherrod Brown, D-Ohio to be "Toss Ups."

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Sen. Joe Manchin, D-W.Va., who holds one of the Democrats' most vulnerable seats, recently announced he would not be seeking re-election, throwing even more focus on the Big Sky State's Senate battle. The Cook Report ranks the blue West Virginia seat as "Solid Republican" going into 2024.