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.

Cook Political Report rating: A toss-up

Ohio: Sherrod Brown faces the fight of his political life

Besides Mr. Tester, Senator Sherrod Brown is the only other Democrat defending a seat in a solidly Republican state. He too has had the advantage of winning in strong Democratic years — 2006, 2012 and 2018 — and like Mr. Tester, he has established an image as a stalwart supporter of the working-class voters who will decide the election. As a powerful member of the Senate — he is the chairman of the Senate Banking Committee — Mr. Brown has amassed a considerable war chest for his re-election campaign.

Unlike Mr. Tester, he will be running against a Republican who was not the Ohio G.O.P. establishment’s choice. Bernie Moreno, instead, was the candidate of Mr. Trump. The Republican’s sizable fortune will seed fund-raising and undergird his campaign against the incumbent, but Democrats boosted Mr. Moreno’s candidacy during the primary because they believe his business background will make him vulnerable to attack.

Cook Political Report rating: A toss-up

Arizona: Kari Lake, a prominent election denier, tries again

The retirement of Senator Kyrsten Sinema, the Democrat-turned-independent iconoclast, has set up a stark Senate race between Representative Ruben Gallego, a progressive, and Kari Lake, a former television news anchor and a favorite of Mr. Trump’s Make America Great Again movement who lost her race for governor in 2022. The primary in Arizona is July 30, but Mr. Gallego and Ms. Lake have established themselves as the odds-on favorites to represent their parties in the open Senate contest.

President Biden narrowly won Arizona in 2020, and unlike Ohio and Montana, the state promises to be a presidential battleground, potentially warping the Senate race. Ms. Lake made a name for herself by falsely claiming that Democrats stole the Arizona election for Mr. Biden in 2020, then falsely claiming that her Democratic opponent in the governor’s race, Katie Hobbs, stole her election. Mr. Gallego is less well-known outside of his Phoenix House district, but as a Latino with a Harvard pedigree and combat experience in Iraq with the Marine Corps, he has a compelling biography.

Cook Political Report rating: A toss-up

Michigan: Trump looms large

The full range of Republican Party factions will fight it out for the right to contend for the Senate seat of Senator Debbie Stabenow, a Democrat who is retiring. There’s Peter Meijer, who voted to impeach Mr. Trump just after being sworn into the House, then lost in the 2022 Republican primary to a Trump-backed challenger. Mr. Meijer has now said he will vote for Mr. Trump in November. There’s Justin Amash, the libertarian-minded former congressman who denounced Mr. Trump, faced a fierce backlash that chased him from his party, tried to run for his House seat as an independent, lost and is now running for the Senate again as an anti-Trump Republican. Then there’s Mike Rogers, the mainstream Republican who said the party needed to move on from Mr. Trump, then wooed and won Mr. Trump’s endorsement for Senate and embraced him.

Mr. Rogers has to be considered the favorite in the Aug. 6 primary, but past and present views of the former president will loom over this race all summer. The primary winner will almost certainly face Representative Elissa Slotkin, a Democrat who has used her national security credentials to win over swing voters in Central Michigan since 2018. Her trick will be to keep those centrist voters and energize more liberal voters in and around Detroit. And looming above it all is the presidential contest.

Cook Political Report rating: Leaning toward Democrats

Nevada: Low-key incumbent vs. political newcomer

In recent years, Nevada Democrats have profited off Republican voters’ penchant for nominating candidates from the G.O.P.’s extremes, but this year, party leaders are rallying around Sam Brown, a political neophyte with an extraordinary story. The West Point graduate nearly died in Kandahar, Afghanistan, when a roadside bomb burned him badly and left him permanently scarred. His thin political résumé could be a plus, since it will make him difficult to label.

Senator Jacky Rosen, the incumbent Democrat, isn’t flashy, but the power of incumbency matters, unless Mr. Biden’s support in the state craters.

Cook Political Report rating: Leaning toward Democrats

Wisconsin: A wealthy Republican candidate faces questions over his ties to the state

Senator Tammy Baldwin, a Democrat, has been a low-key fixture in Wisconsin politics since her election to the State Assembly in 1992. With her comes little drama, but Wisconsin, a state that just re-elected its famously quiet governor, Tony Evers, in 2022, seems to like Democrats who speak softly.

Republicans have recruited Eric Hovde, a banker and businessman who, if nothing else, can finance his own campaign. But his connections to Southern California in a state full of Badger pride have helped keep this race leaning toward the incumbent.

Cook Political Report rating: Leaning toward Democrats

Pennsylvania: David McCormick tries again

The Keystone State may be a key battleground in the presidential election this year, but Senator Bob Casey, the Democratic incumbent, is an institution. His expected Republican opponent is David McCormick, the former chief executive of Bridgewater Associates, one of the largest hedge funds in the world. Mr. McCormick lost the Republican Senate primary in 2022 to Mehmet Oz, and the lines of attack honed two years ago on his wealth and his mansion in Connecticut are sure to be recycled.

Cook Political Report rating: Leaning toward Democrats

Maryland: Larry Hogan makes things interesting

Reliably blue Maryland should not be in play, but Larry Hogan, the moderate former Republican governor, has decided to run for the Senate seat of Ben Cardin, the retiring Democrat, making the race one to watch.

Democrats had figured they could count on the three-term Representative David Trone, the wealthy founder of Total Wine & More, a large retailer of wine and spirits. Then Mr. Trone used a racial slur at a House hearing, for which he has apologized and said was inadvertent. But several Black Democrats endorsed Angela Alsobrooks, the Prince George’s County executive, ahead of the May 14 primary, as did Representative Jamie Raskin of Maryland, the top Democrat on the powerful House Oversight Committee.

Cook Political Report rating: Likely Democratic

Texas and Florida: Democrats try to flip seats in reliably red states

Democrats have only two races to play offense in: Senator Ted Cruz’s campaign in Texas and Senator Rick Scott’s in Florida. Both men have never been personally popular in their states, but those states have been reliably Republican of late. Democrats like their candidates, Representative Colin Allred in Texas and former Representative Debbie Mucarsel-Powell in Florida, but it would most likely take severe erosion of Mr. Trump’s support to put those Senate seats in play.

Cook Political Report ratings: Likely Republican

Bonus races to watch: Long shots

Utah is nobody’s idea of a swing state, and the retirement of Senator Mitt Romney at the end of his term has drawn in nearly a dozen Republicans ahead of the June 25 primary, including Representative John Curtis; Brent Orrin Hatch, the son of Senator Orrin G. Hatch; and Brad Wilson, the former speaker of the State House. But one Democrat made waves with a unique announcement video: Caroline Gleich, a professional ski mountaineer.

Nebraska is almost as red as Utah, with an incumbent Republican, Deb Fischer, running for re-election. Her main opponent is not a Democrat but an independent, Dan Osborn, who led a strike at the Kellogg’s plant in Omaha in 2021 and is testing whether his pro-labor, working-class message can resonate at a time when the union movement is resurgent.

Cook Political Report ratings: Solidly Republican