A single state could determine whether Democrats maintain control of the Senate after the midterm elections, a tenuous advantage that hinges on the tiebreaking vote of Vice President Kamala Harris.

Thirty-four Senate seats are at stake in 2022, but the list of races considered competitive is much smaller.

Most are in states that were fiercely contested by President Biden and former President Donald J. Trump in 2020.

The burden will be on Democrats to try to ward off the midterm losses that have historically bedeviled the party holding the presidency, said Donna Brazile, a former interim party head and veteran strategist.

“Joe Biden has a lot riding on these states,” she said. “He doesn’t have a lot of wiggle room.”

Alaska

Of the seven Republicans in the Senate who voted to convict Mr. Trump in the impeachment trial that followed the Jan. 6 Capitol riot, Senator Lisa Murkowski is the only one facing re-election in 2022.

Mr. Trump, who is seeking to exact revenge against his impeachment foes, endorsed Kelly Tshibaka, a former commissioner of the Alaska Department of Administration, to run against Ms. Murkowski in the primary.

Arizona

Senator Mark Kelly, a Democrat who won a special election in 2020 to fill the seat once held by John McCain, is now seeking a full term.

Both parties are prioritizing the race.

The Republican field includes Mark Brnovich, Arizona’s attorney general since 2015; Mick McGuire, a retired major general in the U.S. Air Force; Jim Lamon, a businessman; and Blake Masters, chief operating officer of an investment firm run by Trump’s tech pal Peter Thiel.

Georgia

Stacey Abrams’s decision to run again for governor could boost the re-election prospects of Senator Raphael Warnock, a fellow Democrat, Ms. Brazile said.

Mr. Warnock, the pastor at the storied Ebenezer Baptist Church in Atlanta, is seeking a full term after defeating Kelly Loeffler last January in a runoff.

His victory helped give Democrats control of both Senate seats in Georgia, where an expansion of the Democratic voter rolls in Atlanta’s suburbs has dented Republicans’ political advantage in the South and flipped the state for Biden.

Now, Mr. Warnock is seeking a full term.

“What he’ll get from Stacey is somebody who can stir up the electorate to get the results he needs to win in 2022,” Ms. Brazile said.

Herschel Walker, the Georgia college football legend backed by Mr. Trump, is the favorite among seven Republicans who have filed to run so far. He has faced repeated accusations of threatening his ex-wife.

Florida

In Mr. Trump’s adopted home state, Senator Marco Rubio is seeking a third term.

Mr. Rubio had raised more than $11.6 million in 2021 through September.

He is facing Representative Val B. Demings, a Democrat with significant name recognition who out-raised him over the same period, with more than $13 million.

Nevada

Catherine Cortez Masto, the first Latina senator, faces her first re-election test since her milestone victory in 2016, a race that was flooded with nearly $90 million in outside spending.

Opposing her is Adam Laxalt, a former Nevada attorney general who ran unsuccessfully for governor in 2018.

Mr. Laxalt has been endorsed by both Mr. Trump and Senator Mitch McConnell, the minority leader. Mr. Biden carried Nevada by fewer than 34,000 votes last year.

New Hampshire

Republicans have circled New Hampshire as a pickup opportunity, salivating over the dismal approval numbers of Senator Maggie Hassan, a Democrat.

But their enthusiasm was tempered when Gov. Chris Sununu said that he would run again for his current office instead of the Senate. Kelly Ayotte, whom Ms. Hassan unseated by about 1,000 votes in 2016, also opted out.

Don Bolduc, a tough-talking Republican candidate and retired Army general, caused a stir recently when he called Mr. Sununu a “Chinese communist sympathizer.”

North Carolina

Senator Richard Burr, another Republican who voted to convict Mr. Trump during his second impeachment trial, is retiring.

Waiting in the wings is a crowded field of Republicans that includes Pat McCrory, a former governor; Representative Ted Budd, who has been endorsed by Trump; and Mark Walker, a former congressman.

The Democrats include Cheri Beasley, a former chief justice of North Carolina’s Supreme Court and the first Black woman to serve in that role, and Jeff Jackson, a state senator and military veteran from the Charlotte area.

Ohio

A large field of G.O.P. candidates will vie for the seat being vacated by the Republican senator Rob Portman, who is retiring.

The leading Republican is Josh Mandel, Ohio’s former treasurer and an ardent Trump supporter. J.D. Vance, the “Hillbilly Elegy” author and Republican venture capitalist who has performed a whiplash-inducing conversion to Trumpism, is also running.

Other G.O.P. candidates include Matt Dolan, a state senator; Jane Timkin, the state party’s former head; and the businessmen Bernie Moreno and Mike Gibbons.

“You’ve got a lot of people fighting for the populist conservative lane,” said Beth Hansen, a Republican strategist and former manager of John Kasich’s campaigns for governor and president.

Ms. Hansen downplayed the possibility of Republicans alienating moderate voters in a combative primary.

“Honestly, I’m not sure these guys could pivot any further to the right,” she said.

Representative Tim Ryan, supported by Ohio’s other senator, Sherrod Brown, is a prohibitive favorite among Democrats.

Pennsylvania

An open-seat race in Pennsylvania generated even more of a buzz when the celebrity physician Dr. Mehmet Oz recently jumped into fray.

He joined a large group of candidates trying to succeed Senator Patrick J. Toomey, a Republican critic of Mr. Trump who is retiring.

Dr. Oz’s entrance came just days after Sean Parnell, a leading Republican endorsed by Mr. Trump, suspended his campaign amid allegations of spousal and child abuse.

Kathy Barnette, a former financial executive, is also running as a Republican, and David McCormick, a hedge fund executive, has been exploring getting into the race as well.

Democrats have several seasoned candidates that include Lt. Gov. John Fetterman and Representative Conor Lamb. Also running are Dr. Val Arkoosh, a top elected official from the Philadelphia suburbs, and Malcolm Kenyatta, a state representative from Philadelphia.

Wisconsin

A top target of Democrats is Senator Ron Johnson, a Republican whose approval rating has cratered amid an onslaught of television ads criticizing him for casting doubts about Mr. Biden’s election.

Mr. Johnson has yet to announce his re-election plans.

The top tier of Democrats includes Mandela Barnes, the lieutenant governor; Sarah Godlewski, the state treasurer; Alex Lasry, the Milwaukee Bucks executive ; and Tom Nelson, the top elected official in Outagamie County.