An academic with a forthcoming biography of 1972 Democratic presidential candidate and former senator George McGovern has confirmed that the South Dakotan fathered a child before he was married.

He said McGovern, as an 18-year-old freshman at Dakota Wesleyan University in Mitchell, S.D., lost his virginity to the girlfriend of a friend during a trip to Lake Mitchell in December of 1940 or January of 1941, and immediately got her pregnant.

McGovern, a decorated World War II pilot and a liberal icon who died in 2012, lost his seat in the U.S. Senate in the Republican sweep of 1980. He served as U.S. ambassador to the United Nations Agencies for Food and Agriculture during the administration of President Bill Clinton.

Rumors — most recently in stories about the South Dakota senator’s FBI file, which included references to an “illegitimate child” — about a child McGovern had out of wedlock before he married Eleanor McGovern in 1943 dogged McGovern for much of his political career. Before McGovern died in 2012, he told Thomas J. Knock, a history professor at Southern Methodist University in Dallas, about the affair and the daughter he had in 1941.

“He told me about it voluntarily about 15 years ago because he wanted me to write about it,” Knock told The Washington Post in a telephone interview. “He felt confident in my credentials as a historian and biographer to deal with it responsibly.”

Though the first volume of Knock’s McGovern biography — “The Rise of a Prairie Statesman,” billed as “the first major biography of the 1972 Democratic presidential candidate” by Princeton University Press — is not yet available, Knock offered details of how the future presidential contender and leading critic of the war in Vietnam became embroiled in a situation that has derailed many a political career.

“It was an utterly unplanned, frenzied episode,” Knock said. “It was his first time. It was over in a few minutes, and they regretted their impulse. A few weeks went by, and she discovered she was pregnant and told him about it. He was seized by remorse and shame.”

McGovern was the son of a Methodist minister, and Knock said, his religious upbringing in a small town didn’t help matters.

“He was just wracked with guilt, causing this girl so much of a problem and a scandal in the offing for his family,” Knock said. “He had several days of paralyzing anxiety, and worse for not being able to unburden himself to parents.”

Knock said the mother of McGovern’s daughter is now dead; he said he knew the identity of the daughter, but was unwilling to reveal it for fear of compromising her privacy. Asked whether the daughter was still alive, Knock said: “I have no reason to think she isn’t living.”

Knock also said that, in a time when the word “bastard” was still in popular use, the young woman McGovern had gotten pregnant took charge, electing to leave town, take refuge with her family and have McGovern’s child in secret.

“She was remarkably calm and strong, decided within a week or so go to Indiana and stay with her older sister and brother-in-law,” Knock said. “She gave birth there.” Hence the name of the McGovern rumor among historians and politicos: “the Fort Wayne story.”

The unnamed mother, Knock said, kept McGovern away from her child — for a time.

“She insisted that neither she nor George intrude on the daughter’s life,” Knock said.

How McGovern’s secret daughter affected his family is not clear. Knock said McGovern’s wife, Eleanor, who died in 2007, found out about the girl in the 1970s — the couple had five children of their own. Knock doesn’t know what Eleanor thought of the situation.

“I never asked Eleanor about this,” Knock said. “I didn’t think it was appropriate. … [McGovern] didn’t write about it in his autobiography. I think she didn’t want him to.”

Eventually, however, the man who wanted to be president met his daughter.

“He gave her presents and stuff like that,” Knock said. “In a way, it’s a sad story, but in the moment … I think the way he rationalized it was that it worked out all right for all concerned.”

In the days of Brad Pitt and Angelina Jolie, among many others, having a child outside of the bonds of holy matrimony is not a big deal. Moreover, McGovern, while arguably the nation’s leading Democrat between the administrations of Lyndon B. Johnson and Jimmy Carter, never actually became president. Other presidents — Thomas Jefferson, Grover Cleveland and Warren G. Harding come to mind — are alleged to have had children out-of-wedlock. And, of course, there’s former South Carolina senator Strom Thurmond.

Why care about the secret offspring of a man who got absolutely shellacked by President Richard Nixon in 1972? Because, in the Watergate-era Nixon White House, it nearly became a campaign issue.

The Fort Wayne story and the secret mother and child behind it did not stop McGovern’s rise. As the decades went by, he married Eleanor; he became a bomber pilot in World War II; an acolyte of Democratic presidential candidate Adlai Stevenson; a congressman. He worked for Kennedy’s Food for Peace program, was elected senator, and tried to fill the hole in the Democratic Party after Robert Kennedy’s assassination in 1968.

But when McGovern set his sights on 1600 Pennsylvania Ave. and ran against Nixon in 1972, the Fort Wayne story became potential ammunition — and remained ammunition as McGovern continued to serve in the Senate as Watergate unfolded. Or, at least, Nixon chief of staff H.R. Haldeman thought so, as Bob Woodward and Carl Bernstein reported in The Post on Aug. 2, 1973.

“As a counter-move to the Senate Watergate hearings, H.R. Haldeman … proposed leaking a story to the press that Sen. George McGovern (D-S.D.) was the father of an illegitimate child,” Woodward and Bernstein wrote. “The idea behind the move, Haldeman suggested in a memorandum, would be to foster the impression that President Nixon and his White House staff had known about the McGovern story but had refused to use it during the 1972 presidential campaign — thus demonstrating ‘that we ran a clean campaign.'”

During Senate Watergate hearings, Haldeman said “the President had refused to let out the Fort Wayne story,” as Woodward and Bernstein wrote. But the revelation that there was a Fort Wayne story led to the next question: Was the story true?

McGovern told Woodward and Bernstein the story referenced a birth certificate in Fort Wayne that listed McGovern as “the father of a child born out of wedlock in the early 1940s.” Then, McGovern lied, denying he was the father, but The Post kept on the trail.

“The Washington Post has confirmed the existence of such a birth certificate, and contacted the child’s mother, who also denied that McGovern was the father,” Woodward and Bernstein wrote. Later stories in 1973 said that McGovern’s name had been erased from the certificate, but an Indiana court prevented the document from becoming public.

How Nixon learned about the Fort Wayne story is not clear. But because the information was in McGovern’s recently unearthed FBI file, some speculated that Nixon was tipped off by FBI director J. Edgar Hoover, who frequently clashed with the liberal senator from South Dakota.

In 1975, as Jonathan Ellis of the Sioux Falls Argus Leader reported, McGovern, with his eyes on running for the White House in 1976, met with the FBI to discuss any information about his alleged secret daughter that might appear in his file.

Investigators “shared with McGovern that in 1960, during a thorough background search after he’d been nominated to serve in John F. Kennedy’s administration, FBI investigators verified an allegation that McGovern had fathered a child,” Ellis, who obtained McGovern’s FBI file, wrote. “McGovern, according to the FBI’s account of the meeting, ‘made no comment nor asked any questions about the statement that the allegation concerning the illegitimate child had been verified during the special inquiry investigation.'”

The story remained squashed.

“It was a rumor that followed in the shadows of his political career and one that his presidential campaign braced itself to respond to if it ever became public,” Ellis wrote. “Somehow, the material ended up with President Richard Nixon’s re-election campaign – possibly leaked by the bureau’s longtime director, J. Edgar Hoover – but it was never disclosed during the campaign.”

Knock, the historian in whom McGovern confided, said Nixon didn’t need the Fort Wayne story to beat McGovern. After Thomas Eagleton, McGovern’s running mate, revealed he had been treated for depression and left the ticket, defeat was already in the air.

“[Nixon’s special counsel Charles] Colson and Haldeman were attempting to convey the impression that the Nixon administration wasn’t quite as odious as the liberal media thought they were,” Knock said. “But that became kind of a moot point because of the Eagleton affair. They didn’t need to do that. They talked through every possible angle and came to the conclusion that [using the Fort Wayne story] would make them look bad. Why bring this up after he’s not going to win after the Eagleton albatross?”

Knock also said that history should remember the great man behind a scandal that, almost 80 years later, hardly looks like a scandal.

“These are the circumstances that confront teenagers,” Knock said. “It’s commonplace. It could have happened to almost anyone.”

But somehow, one of the leading American politicians of the second half of the 20th century kept his greatest secret a secret.

“He kept it to himself for about 35 years,” Knock said.

Correction: An earlier version of this article referred to McGovern as a “fighter pilot.” He was a bomber pilot.