The nonpartisan Cook Political Report shifted Montana’s pivotal Senate race from “toss-up” to “lean Republican,” signifying the GOP is a favorite to win in the Treasure State.

Sen. Jon Tester, Montana Democrat, and his Republican opponent, Tim Sheehy, are locked in a tight race that Republicans and Democrats see as the key to power in the upper chamber. 

The change comes after Mr. Tester began this election cycle as the favorite. Mr. Sheehy’s ability to consolidate voters on the side of former President Donald Trump, who is wildly popular in the state, shows the political winds moving against the three-term incumbent. 

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“This means that Republicans are now an even heavier favorite to win back control of the Senate, regardless of the result at the top of the ticket,” the nonpartisan election tracker said. 

With Sen. Joe Manchin, West Virginia independent, announcing his retirement at the end of this year, the GOP considers that state an easy win. That means Montana, where Mr. Tester has consistently survived tough races in an increasingly conservative state, will determine the balance of power in the Senate.

The Cook Political Report’s change reflects the University of Virginia Center for Politics, which also changed its outlook of the race to “lean Republican.” On top of those takes, AARP’s bipartisan poll found Mr. Sheehy led Mr. Tester 51%-45%, and 49%-41% when including third-party contenders.

Mr. Tester isn’t giving up. His campaign is again making a bid to capture Montana’s batch of independent and split-ticket voters, and internal polling from Republicans and Democrats shows a tighter race than public polls disclose.