(Updates with polls closing)
By Nathan Layne
ATLANTA, Dec 6 (Reuters) - Polls closed on Tuesday in a hard-fought runoff election between Democratic Senator Raphael Warnock and Republican former football star Herschel Walker that will determine whether Democrats can expand their razor-thin Senate majority.
The race is a last test of Donald Trump's clout with midterm voters as he seeks the Republican nomination to challenge President Joe Biden in 2024. The former president, who endorsed Walker, had a mixed record in his most competitive endorsements for Congress in this year's elections.
Walker's campaign has been plagued by repeated gaffes and a variety of allegations, including claims by former girlfriends that he paid for their abortions, even though he has campaigned for the procedure to be outlawed. He has denied the accusations.
Retired firefighter Keith Fields, 58, showed up an hour before polls opened in Smyrna to cast his ballot for Walker because of his stance on immigration, taxes and abortion without exceptions.
"It's not so much Herschel as the man - but I do like the man - but I'm also voting for the policies that he is going to vote for," Fields said.
The runoff set early voting records in Georgia in a race that has become the most expensive of the 2022 U.S. midterm election season, with more than $400 million spent so far. The contest went to a runoff after neither candidate secured 50% of the vote on Nov. 8.
A victory by Warnock would give Democrats a 51-seat majority in the 100-seat Senate, which would make it slightly easier to advance Biden's nominees for judicial and administrative posts. Most legislation would still require Republican support.
A Walker win, on the other hand, would keep the Senate at its current 50-50 split, with Democrats controlling the chamber thanks to Vice President Kamala Harris's tie-breaking vote.
On a wider scale, a Warnock victory could solidify Georgia as a battleground in presidential elections. It also would be another midterm defeat at the polls for a protege of Trump, who spurred Walker to run.
The U.S. Justice Department said it was sending monitors to polling places to ensure Georgia complies with federal voting rights laws. The department has expressed concerns about possible violations of voting rights in Georgia.
At least 1.9 million people cast their votes before Election Day, equal to 47% of the Nov. 8 turnout, and state election official Gabe Sterling has said he expected 1 million more would vote on Tuesday.
Analysts say the early votes likely tilted Democratic, which means Walker will need strong Election Day turnout from his supporters to overcome the gap.
"We should not rest on our laurels. The job is not done. The truth is my opponent could still win this election," Warnock told a crowd in Norcross, Georgia, on Tuesday.
Warnock is pastor of the historic Atlanta church where assassinated civil rights leader Reverend Martin Luther King Jr. preached. Both Warnock and Walker are Black.
Along with the abortion claims, former girlfriends have accused Walker of domestic abuse. He has also faced accusations that he maintains his primary residence in Texas, not Georgia.
Walker has denied the charges, but they may have discouraged some Republican voters. Warnock edged him 49.44% to 48.49% in November, even as Republican Governor Brian Kemp and other statewide Republican candidates easily won re-election.
Cobb County resident Guri Ben-Hashal, 60, who works in real estate, said he cast a vote against Walker. "I feel that Herschel Walker is unsuitable for the job, to say the least," he said.
At a diner in Marietta, Walker deflected questions about his character, which Warnock has made a centerpiece of his campaign.
"Right now I put my character up against Raphael Warnock any day," Walker said on Tuesday.
This is the third Senate runoff in two years in the state - and the second for Warnock, who first won the seat in a runoff in January 2021. Republicans won a narrow majority in the U.S. House of Representatives in the Nov. 8 election, but fell short of the "red wave" that some in the party had forecast, despite flagging approval ratings for Biden.
In a Reuters/Ipsos poll finished on Tuesday, Biden's approval rating dipped to 38%, down 2 points from a week earlier. The poll has a credibility interval -- a measure of precision -- of 4 percentage points either way.
While Biden did not campaign for Warnock, former President Barack Obama rallied for him in Atlanta last week.
(Reporting by Nathan Layne; Additional reporting by Richard Cowan, Sarah N. Lynch, Doina Chiacu, Moira Warburton and Steve Holland, writing by Andy Sullivan; Editing by Scott Malone, Alistair Bell and Rosalba O'Brien)