New polling suggests that Republicans are poised to gain control of the U.S. Senate next year as a vital toss-up race in Montana shows a lean to the right.

According to a new analysis by The Cook Political Report, GOP challenger Tim Sheehy has gained a slight lead against Democratic incumbent Jon Tester. The shift in voter opinion leaves Republicans poised to win a majority in the Senate in November.

“Successive public polls have shown Sheehy opening up a small but consistent lead,” the Cook analysts say. “Democrats push back that their polling still shows Tester within the margin of error of the race, and that those are exactly the type of close races he’s won before. Tester, however, has never run on a presidential ballot in a polarized environment of this kind before — and even with his stumbles, Sheehy is still the strongest, best-financed candidate he’s ever faced.”

The shift from a toss-up to “leans Republican” comes after a recent AARP poll showing the former Navy SEAL leads with a six-point advantage — at 51 percent — in his race with Senator Tester.

“Beyond the polling, history and recent trends are just not on Tester’s side, as we have mentioned previously,” a Managing Editor with the University of Virginia Center for Politics, Kyle Kondik, writes. “He is one of a relatively small number of partisan outliers in either chamber of Congress, holding a Senate seat that the other party won by 16 points in the most recent presidential election.”

“Perhaps ticket-splitting returns in force this year — if it did, Tester could still survive. But the longer-term trend is clearly toward less ticket-splitting.”

According to the Cook analysis, if Democrats lose the seat in Montana, they will have to sweep toss-up elections in Ohio, Michigan, and states like Texas, Florida, or Nebraska to compensate for the loss.