10 Senate Races to Watch in 2024

The fight for Senate control is playing out almost entirely in Democratically held seats this year as President Biden’s party defends a slim 51-49 seat majority.

The retirement of Senator Joe Manchin III, a Democrat, in deep-red West Virginia has all but ceded one seat to the Republicans, who are targeting a number of vulnerable Democratic incumbents in red or swing states. And if former President Donald J. Trump wins the White House, one seat is all the G.O.P. needs to flip the chamber. Should the Senate come down to a 50-50 split, the vice president plays tiebreaker.

For Democrats to hold the Senate, the party would most likely need all their incumbents to win; for their candidates to prevail in open seats in Arizona, Michigan and Maryland; and for Mr. Biden to be re-elected so Vice President Kamala Harris would play the tiebreaker in an evenly split chamber. The party is targeting two Republican-held seats, but those are considered more difficult terrain.

Here are the Senate races to watch in 2024.

Montana: Farmer vs. former Navy SEAL

Senator Jon Tester, the flat-topped farmer from Big Sandy, Mont., has defied the odds before in his increasingly Republican state, but his Senate victories in 2006, 2012 and 2018 all came in strong Democratic years nationally. His fight for a fourth term will be considerably tougher with Mr. Biden at the top of the ticket in a state that Mr. Trump won by 16 percentage points in 2020. And Mr. Tester will most likely be battling the Republican Party’s selected candidate, Tim Sheehy, a decorated former Navy SEAL and businessman with the wealth to self-finance his campaign, as well as Mr. Trump’s backing.

Mr. Tester has the power of incumbency, and the authenticity of a third-generation Montanan. In 2012, President Barack Obama received 41.7 percent of the vote. Mr. Tester earned 48.6 percent. He may need even more ticket-splitters — people who will vote for Mr. Trump for president and him for Senate — this November.