Control of the U.S. Senate appears likely to flip from the Democratic Party to the Republican Party this fall, as one of the nation’s most endangered Democrats, Sen. Jon Tester of Montana, trails his Republican challenger in his bid for reelection, according to a new poll from The New York Times and Siena College.

Tester, who first won election to the Senate in 2006, is winning over moderate and independent voters and running far ahead of the Democrat at the top of the ticket, Vice President Kamala Harris. But that does not appear to be enough to survive in Montana, a conservative state where former President Donald Trump is ahead by 17 percentage points and where control of the Senate hangs in the balance.

Tim Sheehy, a wealthy Republican businessman and a former Navy SEAL who has never held public office, leads Tester 52% to 44%, the poll shows. Sheehy’s lead is a 7-point advantage without rounding.

Democrats currently hold a 51-seat Senate majority. But with Republicans already set to pick up a seat after the retirement of Sen. Joe Manchin, I-W.Va., who caucuses with Democrats, the party cannot afford to lose additional seats.

The party’s only hope is to secure a 50-50 split and to have Harris win the White House, allowing her running mate, Gov. Tim Walz of Minnesota, to provide the tiebreaking vote as vice president.

At least seven other Democratic-held Senate seats are competitive this fall, including in the presidential battlegrounds of Arizona, Michigan, Pennsylvania, Nevada and Wisconsin. In late September, a series of Times/Siena surveys in four of those races, as well as in Ohio, found Democrats ahead, though narrowly in some cases.

The problem is that the Democratic Party has scarce opportunities to flip any Republican-held seats in 2024 to make up for any potential losses, such as in Montana.

The best opportunity, according to new Times/Siena polling, may be in Texas, which Democrats have long dreamed of flipping but where they have fallen well short in recent years. Sen. Ted Cruz, a Republican seeking his third term, leads his Democratic challenger, Rep. Colin Allred, 48% to 44%, according to a Times/Siena poll in Texas.

And in Florida, a Times/Siena poll found that Sen. Rick Scott, the Republican incumbent, is comfortably ahead of Rep. Debbie Mucarsel-Powell, his Democratic rival, by an even wider margin, 49% to 40%.

Both Cruz and Scott lead by smaller margins in their states than Trump is ahead of Harris.

A third potentially competitive Republican-held seat has emerged in recent weeks, though the reelection bid of Sen. Deb Fischer, R-Neb., was not included in the polls. Fischer is running against an independent, Dan Osborn, and Republicans have recently come to her campaign’s aid with advertising.

In Montana, money has been flooding the state for months. With a population of around 1.1 million, Montana is set to see more than $265 million in television spending related to the Senate race, according to AdImpact, the ad-tracking service.

Republicans have circled Tester’s race as a top priority in 2024 largely because the state has become so solidly Republican in national politics. Trump won there with 57% of the vote in 2020 — the same percentage he was pulling in the poll.

Tester, with his flattop haircut and seven fingers — he lost three digits in a meat-grinding accident as a child — has cemented a distinctive image that has long allowed him to outrun his party label and win reelection in 2012 and 2018.

In the poll, 55% of likely voters said they would prefer that Republicans control the Senate, compared with only 37% who prefer Democratic control.