This spring, Obama's links to Ayers briefly became a campaign controversy. Now American Issues Project is spending $2.8 million to air a TV ad highlighting links between Obama and Ayers, a founder of the Weather Underground Organization, which opposed the Vietnam War and was responsible for several bombings.

Obama released a rebuttal TV ad Monday. "With all our problems, why is John McCain talking about the '60s, trying to link Barack Obama to radical Bill Ayers?" a narrator asks.

A movie, Hype: The Obama Effect, was first shown Sunday in Denver. It was made by Citizens United, another conservative group, and explores the Ayers-Obama connection and questions whether Obama can unite the country.

Documents released today by the University of Illinois at Chicago (UIC) will be scrutinized for clues to the relationship. 

Ayers was a founder of the Chicago Annenberg Challenge, a school-reform group. Obama chaired its board from 1995-99. National Review reported last week that UIC said records detailing meetings and other business were public, then reversed itself. UIC said Friday there was a misunderstanding.

Obama and Ayers, now a professor and author, live a few blocks apart in this city's Hyde Park neighborhood. Conservative activists say their relationship is evidence that Ayers' radical politics helped mold Obama's views.

"Ayers is clearly a relevant issue as it relates to Obama's pattern of relationships," says David Bossie of Citizens United.

American Issues Project spokesman Christian Pinkston says Ayers' influence is an open question, but "it's hard to see how one actually could resolve having any sort of relationship with an admitted, remorseless domestic terrorist."

The ad notes that Weathermen bombed the Capitol and asks why Obama would "be friends with someone who bombed the Capitol and is proud of it?"

Ed Failor, a founder of American Issues Project, worked for John McCain's Iowa campaign.

The Obama campaign on Monday released a letter sent to the Justice Department last week asserting that the American Issues Project ad violates federal rules that bar tax-exempt political groups from advocating a candidate's election or defeat. Pinkston called it "a sad ploy to circumvent the First Amendment." The campaign also released a letter sent last week to TV stations disputing the ad's truthfulness.

Campaign officials say the 47-year-old candidate and the 63-year-old UIC education professor have only a casual relationship.

"The last time Obama saw Ayers was about a year ago when he crossed paths with him while biking in the neighborhood," says Ben LaBolt, a campaign spokesman. "The suggestion that Ayers was a political adviser to Obama or someone who shaped his political views is patently false."

How their paths crossed

When Obama was asked about Ayers in an April debate, he said, "the notion that … 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."

After that debate, Chicago Mayor Richard Daley released a statement saying he doesn't condone what Ayers did in the 1960s. "It was a difficult time, but those days are long over," he said.

Ayers and Obama have moved in some of the same circles:

•In 1995, Ayers hosted a brunch for Obama, who was running for the Illinois Senate.

The ad says this meeting launched Obama's political career. Quentin Young, a physician who was there, says it was a typical Hyde Park event and to imply otherwise is "guilt by simultaneously being in the same place."

•In 1997, they were on a juvenile justice panel sponsored by the University of Chicago. They were on a 2002 panel on intellectualism that was co-sponsored by the Chicago Public Library.

•In 1997, the Chicago Tribune published a blurb from Obama about books he was reading. Obama said he was reading Ayers' A Kind and Just Parent: The Children of Juvenile Court.

•From 1999-2002, both men were on the board of the Woods Fund, a Chicago foundation that makes grants to arts and civic groups. Obama left the board in 2002; Ayers remains on it.

Laura Washington, chairwoman of the Woods Fund board, says suggestions of close ties are "an attempt to demonize Bill as a way of damaging Barack Obama."

•Ayers gave $200 to Obama's 2001 state Senate campaign.

No regrets or apologies?

Ayers did not respond to interview requests. Federal charges for crossing state lines to incite riots and conspiracy were dropped because of prosecutorial misconduct. He was in hiding for years after three Weathermen died in 1970 when bombs they were making exploded.

In a New York Times story published by coincidence on Sept. 11, 2001, about his memoirs, Fugitive Days, he said, "I don't regret setting bombs. … I feel we didn't do enough." After that comment was raised in the April debate, Ayers posted his 2001 reply to the New York Times story on his blog. "I said I had a thousand regrets, but no regrets for opposing the war with every ounce of my strength," he wrote.

In March, Ayers wrote on his blog about demands that he apologize for his past: "In some part, apologizing is rejecting."

Ayers is married to Bernardine Dohrn, who was once on the FBI's 10 Most Wanted List for inciting a riot and conspiracy. She is an associate law professor at Northwestern University.

Tom Hayden, an anti-war activist who met Ayers in the 1960s and later was elected to the California Legislature, says Ayers' past should be forgiven.

"I have met and like John McCain, but he bombed, and presumably killed, many people in a war I opposed," Hayden says. "If I can set all that aside, I would hope that Americans will accept" that Ayers has changed, too.

McCain asked after April's debate how Obama can "countenance someone who was engaged in bombings." In May, McCain said his campaign "is not going to be about" Ayers nor other Obama associates.

Cass Sunstein, a University of Chicago law professor who knows both men, is "very disturbed by (Ayers') past and by his refusal to disavow what he did." Still, he says, "I think the implications of this for Obama are zero."