WASHINGTON — Once again, a presidential campaign is refighting the 1960s.
Questions about Barack Obama's relationship with a '60s radical named William Ayers are only the latest in a long line of clashes over the turbulent decade that have troubled campaigns ranging from Dan Quayle's to Bill Clinton's and George W. Bush's.
"We're getting dragged right back into Vietnam again," said Dennis Goldford, a political scientist at Drake University in Iowa. "American politics for the last 40 years has been an argument over the 1960s."
There's a difference this time: Unlike all those baby boomers, Obama was a kid at the time. Nonetheless, his apparently casual relationship with Ayers since then still threatens to drag him into the dangerous political crosscurrents of the time.
To many Americans, the '60s are ancient history, and the names of radicals such as Ayers, who protested the Vietnam War with bombs, a forgotten footnote to it. Some 186 million Americans, or 63 percent, weren't born or were under the age of 6 in 1970.
But as Obama may well learn, the story will be told and retold throughout his campaign; it's already a staple of conservative news outlets.
Obama was pressed about Ayers during a debate in Philadelphia on Wednesday, first by ABC News moderators, then by his rival for the Democratic presidential nomination, New York Sen. Hillary Clinton.
The key question was Obama's relationship with a man who helped lead a radical group initially called the Weathermen, then the Weather Underground, in the late '60s and early '70s. It used violence to try to undermine the U.S. government and force an end to the Vietnam War.
The group took its name from a 1965 Bob Dylan song, "Subterranean Homesick Blues," a key line being: "You don't need a weatherman to know which way the wind blows."
The only people whom the Weathermen's bombs ever killed were three of their own, who died when one of the bombs they apparently were assembling accidentally blew up a New York townhouse in 1970. One account said the bomb was destined for Fort Dix, N.J.
Ayers wasn't present. He and other members went into hiding after that, and he met and became romantically involved with a better-known radical, Bernadine Dohrn.
The group detonated bombs at the New York City Police headquarters in 1970, the U.S. Capitol in 1971 and a Pentagon bathroom in 1972.
Dohrn and Ayers turned themselves in to authorities in 1980. Federal charges against them were dropped because of government misconduct in the case. Dohrn faced state charges of mob action, fleeing prosecution and kicking a police office in the groin.
She was fined $1,500 and placed on probation. Defiant, she issued a statement that "I remain committed to the struggle ahead. I regret not at all our efforts to side with the forces of liberation."
Ayers wasn't charged.
The two, now with children, became part of the Chicago intellectual community. He became a professor of education at the University of Illinois at Chicago and active at the Woods Fund, a charity devoted to helping the poor.
In a memoir titled "Fugitive Days," Ayers looked back on his days in the Weather Underground.
"Of all those fugitive days — of all those terrible, exquisite years — I regret nothing for myself," he wrote. "I can't quite imagine putting a bomb in a building today — all of that seems so distinctively a part of then. But I can't imagine entirely dismissing the possibility either."
Ayers also said that he didn't regret setting bombs in an interview with The New York Times. That interview was published on Sept. 11, 2001, hours before terrorists attacked the World Trade Center. Clinton tried to link the two events in Wednesday's debate, though, of course, neither Ayers nor The Times could have known that al Qaida terrorists would strike later that day.
Ayers has had occasional contact with Obama in Illinois.
When Obama first ran for the Illinois Senate in the mid-1990s, Ayers and his wife hosted a reception at their home for Obama to meet other Democrats. Ayers contributed $200 to Obama's state Senate campaign in 2001, but nothing to his U.S. Senate or presidential campaigns.
They also overlapped on the board of the Woods Fund for three years before Obama left it in 2002.
In the debate Wednesday night, Obama called Ayers' radical violence "detestable" and insisted that they aren't close.
He also insisted that he shouldn't be held politically accountable for the violence of the '60s.
"The notion that somehow as a consequence of me knowing somebody who engaged in detestable acts 40 years ago when I was 8 years old, somehow reflects on me and my values, doesn't make much sense," Obama said.
Sense or not, it's become an issue, and he likely hasn't heard the last of it, as the '60s battles grind on and on.