Denver - Sen. Barack Obama's campaign and its allies, mindful of the lessons of the Swift boat attacks of 2004, have begun an aggressive, multi-pronged attack on an advertisement running in swing states that seeks to link the Democratic presidential candidate to former domestic terrorist William Ayers.

    With threats of legal action, boycotts and a response ad launched quietly to avoid publicity, the Obama campaign has put conservative donors and television stations on notice that 2008 will not be 2004, when Sen. John F. Kerry, the Democratic nominee, waited weeks to respond to attacks on his Vietnam War record and ultimately did so ineffectively. Christian Pinkston, a spokesman for the American Issues Project, which is airing the anti-Obama ad, called the response intimidation and harassment.

    Obama campaign lawyer Robert F. Bauer replied: "If someone rides up to a convenience store with a sawed-off shotgun and a prior record, I'm not intimidating anybody by calling the cops. . . . If this [Republican] campaign is going to be run in McCarthyite fashion by lawbreakers in an illegal way, they are going to pay a price."

    Ayers was a member of the Weather Underground, a radical organization that claimed responsibility for a dozen bombings between 1970 and 1974. He is now a professor at the University of Illinois at Chicago and an expert on public school reform.

    The ad, financed with a $2.9 million donation from Texas billionaire Harold Simmons, a fundraising bundler for Sen. John McCain's Republican campaign, says that Obama has defended Ayers as "respectable" and "mainstream" and that he launched his political career from Ayers's home. The Obama campaign says the assertions are demonstrably false.

    "Why would Barack Obama be friends with somebody who bombed the Capitol and is proud of it?" intones a voice on the ad, which is running in conservative areas of Michigan, Ohio, Pennsylvania and Virginia. "Do you know enough to elect Barack Obama?"

    The ad is no video stunt, said Evan Tracey of the Campaign Media Analysis Group, which tracks political advertising. It began running last Thursday, and as of Tuesday, $360,000 had been spent on 264 showings, 52 of them in the Grand Rapids, Mich., media market, just under 40 around Cincinnati, 18 in Norfolk, and half a dozen around Pittsburgh, a corner of Pennsylvania that Sen. Hillary Rodham Clinton dominated in the spring Democratic primary battle.

    "Certainly it connects with base voters," Tracey said. "If you can't get excited about voting for McCain, these are the kinds of ads that get them excited about voting against Obama."

    The television spot has left Obama with the same dilemma that Kerry faced four years ago: Respond and risk pushing the issue onto the political talk shows and front pages of newspapers, or ignore it and hope the attack will not sink in. Kerry, convinced that few would believe that a decorated Vietnam combat veteran would fabricate his war record, chose to ignore the group Swift Boat Veterans for Truth. When he finally responded, his national advertisement stoked the issue all over again.

    Obama campaign officials are taking a different tack. They are running an ad to counter the Ayers spot in the same media markets, but they did so with no national media announcement or news conference. The ad itself blames McCain for the independent Ayers campaign, accusing him of "talking about the '60s" and crimes Ayers committed when Obama was 8 years old, instead of war spending, economic crisis and tax breaks for companies that send jobs overseas.

    Simmons, who helped finance the 2004 Swift boat campaign, has raised $50,000 to $100,000 for McCain's bid, according to the campaign's Web site. Ed Failor Jr., who sits on the American Issues Project board, was an adviser and paid consultant for the McCain campaign. A news release announcing Failor's joining of the McCain presidential exploratory committee was recently removed from the candidate's Web site.

    The counter ad is only part of the Obama team's response. Bauer has written legal letters to television stations, asserting that the Ayers ad is illegal and false, and that its airing is subject to a Federal Election Commission penalty. Obama did not call Ayers "respectable" and "mainstream," Bauer said. Those words were used by a journalist in an article that was posted along with many others on the Obama campaign Web site.

    McCain campaign spokesman Brian Rogers called it "100 percent misleading" to blame the Republican for an ad he had nothing to do with. He said the campaign has had no discussions with Simmons about the ad, the issue or the organization. Failor has not been involved in the McCain campaign for more than a year, Rogers said.

    But Rogers made no effort to distance McCain from the Ayers issue. "If he thinks his long association with an unrepentant domestic terrorist is nothing the American people should be concerned about, he's delusional or naive," Rogers said of Obama. "The guy's running for president. It's an issue."

    Ayers did hold a gathering for him in 1995 when Obama first ran for the Illinois Senate, and he later contributed $200 to his reelection campaign. But Bauer said that hardly constitutes launching the political career of a University of Chicago Law School lecturer and the first African American president of the Harvard Law Review, who had just published his first memoir, "Dreams From My Father."

    A Bauer letter to Deputy Assistant Attorney General John C. Keeney challenges him to make good on a promise to vigorously act in the face of "a knowing and willful attempt to evade the strictures of federal election law."

    More than 93,000 pro-Obama e-mails have flooded Sinclair Broadcasting Group stations that are running the ad, many of them threatening to boycott the stations and their advertisers. Obama campaign spokesman Tommy Vietor warned that other stations that accept the ad can expect the same response.

    Efforts to stop the Ayers ad have not come only from the Obama campaign. A film company in Berkeley, Calif., that made an Oscar-nominated documentary in 2004 on the Weather Underground group has issued a cease-and-desist letter to the American Issues Project, saying that it illegally appropriated copyright images from the film for the ad. Brook Dooley, an attorney for the Free History Project, said shots of Ayers speaking into a camera in an interview and the aftermath of a Weather Underground bombing were copyrighted. The group has informed about 150 stations in Ohio and Michigan of its objection, but Dooley said no decisions have been made about legal action.

    Separately, a new effort by Democratic strategist Tom Matzzie, called Accountable America, is aimed at warning conservative donors of the legal thicket they may be entering by financing independent attack ads like the Ayers spot. He said his group's first target is Simmons.

    Pinkston said his group and his donors are undaunted.

    "The Obama campaign has raised this to front-page news, frankly, with this response ad, with these legal attacks, with their outreach to reporters," he said.