ATLANTA — Senator Warnock defeated Republican challenger Herschel Walker in a Georgia runoff election Tuesday, ensuring Democrats an outright majority in the Senate for the rest of President Biden’s term and helping cap an underwhelming midterm cycle for the GOP in the last major vote of the year.

With Reverend Warnock’s second runoff victory in as many years, Democrats will have a 51-49 Senate majority, gaining a seat from the current 50-50 split with senator-elect John Fetterman’s victory in Pennsylvania. There will be divided government, however, with Republicans having narrowly flipped House control.

Mr. Walker joins failed Senate nominees Dr. Mehmet Oz of Pennsylvania, Blake Masters of Arizona, Adam Laxalt of Nevada, and Don Bolduc of New Hampshire as Trump loyalists who ultimately lost races that Republicans once thought they would — or at least could — win.

In last month’s election, Reverend Warnock led Mr. Walker by 37,000 votes out of almost 4 million cast, but fell short of the 50 percent threshold needed to avoid a runoff. 

Mr. Walker, a football legend who first gained fame at the University of Georgia and later in the NFL in the 1980s, was unable to overcome a bevy of damaging allegations, including claims that he paid for two former girlfriends’ abortions.

The Democrats’ Georgia victory solidifies the state’s place as a Deep South battleground two years after Reverend Warnock and a fellow Georgia Democrat, Senator Ossoff, won 2021 runoffs that gave the party Senate control just months after Mr. Biden became the first Democratic presidential candidate in 30 years to carry Georgia. 

Voters returned Mr. Warnock to the Senate in the same cycle they reelected Governor Kemp, a Republican, by a comfortable margin and chose an all-GOP slate of statewide constitutional officers.

“I’ll work with anyone to get things done for the people of Georgia,” Reverend Warnock, the state’s first Black senator, said throughout his campaign, a nod to the state’s historically conservative lean and his need to win over GOP-leaning independents and at least some moderate Republicans in a midterm election year.

Reverend Warnock, 53, paired that argument with an emphasis on his personal values, buoyed by his status as senior pastor of Atlanta’s Ebenezer Baptist Church, where civil rights leader Martin Luther King Jr. once preached.

Mr. Walker’s defeat bookends the GOP’s struggles this year to win with candidates cast from President Trump’s mold, a setback as he builds his third White House bid.

The Democrats’ new outright majority in the Senate means the party will no longer have to negotiate a power-sharing deal with Republicans and won’t have to rely on Vice President Harris to break as many tie votes.

About 1.9 million runoff votes were cast by mail and during early voting, while the state was on track for a robust Election Day, with state officials estimating an additional 1.4 million votes cast — slightly more than in the November midterm and the 2020 election.

Mr. Walker benefited during the campaign from nearly unmatched name recognition from his football career, yet was dogged by questions about his fitness for office and allegations of hypocrisy.

He acknowledged during the campaign that he had fathered three children out of wedlock whom he had never before spoken about publicly. 

The mother of one of those children told the Daily Beast that Mr. Walker had not seen his young son since January 2016 and had to be taken to court for child support — in direct conflict with Mr. Walker’s years spent criticizing absentee fathers and his calls for Black men, in particular, to play an active role in their kids’ lives.

Mr. Walker, meanwhile, sought to portray Reverend Warnock as a yes-man for Mr. Biden. He sometimes made the attack in especially personal terms, accusing Reverend Warnock of “being on his knees, begging” at the White House.

“My opponent is not a serious person,” Reverend Warnock said during the runoff campaign. “But the election is very serious. Don’t get those two things confused.”

Reverend Warnock promoted his Senate accomplishments, touting a provision he sponsored to cap insulin costs for Medicare patients. He hailed deals on infrastructure and maternal health care forged with Republican senators, mentioning those GOP colleagues more than he did Mr. Biden or other Washington Democrats.

Reverend Warnock distanced himself from Biden, whose approval ratings have lagged as inflation remains high. After the general election, Mr. Biden promised to help Reverend Warnock in any way he could, even if it meant staying away from Georgia. 

Bypassing the president, Reverend Warnock decided instead to campaign with President Obama in the days before the runoff election.

Mr. Walker, meanwhile, avoided campaigning with Mr. Trump until the campaign’s final day, when the pair conducted a conference call Monday with supporters.