DUNWOODY, Georgia — Republican Senate nominee Herschel Walker, who has vehemently opposed abortion rights, paid for an abortion for his girlfriend in 2009, according to a report published late Monday. The candidate called the accusation a “flat-out lie” and said he would sue.

The Daily Beast spoke to a woman who said Mr. Walker paid for her abortion when they were dating. The news outlet reviewed a receipt showing her $575 payment for the procedure, along with a get-well card from Walker and her bank deposit records showing the image of a $700 personal check from Walker dated five days after the abortion receipt.

The woman said Mr. Walker encouraged her to end the pregnancy, saying that the time wasn’t right for a baby, the Daily Beast reported.

In a statement, Mr. Walker said he would file a lawsuit against the news outlet on Tuesday.

“This is a flat-out lie — and I deny this in the strongest terms possible,” he wrote.

The politics editor for the Daily Beast, Matt Fuller, tweeted in response: “I can tell you we stand behind every word and feel very solid about the story.”

Later Monday night, Mr. Walker appeared on Sean Hannity’s program on Fox News, where Mr. Walker was asked if he recalled sending a $700 check to a girlfriend.

“Well, I sent money to a lot of people,” he said. “I give money to people all the time because I’m always helping people. I believe in being generous. God has blessed me. I want to bless others.”

The allegation against Mr. Walker is the latest in a series of stories about the football legend’s past that has rocked the first-time candidate’s campaign in one of the most competitive Senate races in the country. Earlier this year, Walker acknowledged reports that he had three children he had not previously talked about publicly.

As a Senate hopeful, Mr. Walker has supported a national ban on abortions with no exceptions for cases involving rape, incest or a woman’s health being at risk — particularly notable at a time when Roe v. Wade has been overturned by the Supreme Court and Democrats in Congress have been discussing codifying abortion rights into federal law.

“I’m for life,” Mr. Walker has said repeatedly as he campaigns. When asked about whether he’d allow for any exceptions, he has said there are “no excuses” for the procedure.

As the Republican nominee, Mr. Walker has sidestepped many questions about his earlier support for a national abortion ban, instead trying to turn the issue against his Democratic rival, Senator Warnock, who supports abortion rights. 

Mr. Walker often characterizes abortion as “a woman killing her baby” and says he doesn’t understand how Reverend Warnock, a Baptist pastor, can support the procedure being legal.

Campaigning in Dunwoody, an Atlanta suburb, on Monday night, Reverend Warnock stressed his support for abortion rights.

“I have a profound reverence for life. I have a deep and abiding respect for choice. I believe a patient’s room is too small and cramped a space for a woman, her doctor and the United States government,” he said, emphasizing Mr. Walker’s support for a national ban.

Reverend Warnock was dismissive when told of the Daily Beast story and when asked whether it might affect the outcome in Georgia. “I’ll let the pundits decide,” he said.

Mr. Walker’s son, Christian Walker, criticized his father in a series of tweets late Monday, saying his family had “asked him not to run for office.”

“I don’t care about someone who has a bad past and takes accountability,” Christian Walker tweeted. “But how DARE YOU LIE and act as though you’re some ‘moral, Christian, upright man.’ You’ve lived a life of DESTROYING other peoples lives. How dare you.”