The Obama campaign is airing a television ad defending the presumptive Democratic nominee against Republican attacks over his ties to an unrepentant Vietnam War-era terrorist, William Ayers. The 30-second spot marks a response to a new ad by a conservative group, the American Issues Project, which hits Senator Obama for his connections to Mr. Ayers, a former member of the radical group the Weather Underground who is now a professor in Illinois and lives near Mr. Obama. Mr. Ayers once held a fund-raiser for Mr. Obama's state Senate campaign in Illinois, and the Obama campaign has described their relationship as "friendly," although Mr. Obama has denounced Mr. Ayers's association with the Weathermen. "With all our problems, why is John McCain talking about the '60s, trying to link Barack Obama to radical Bill Ayers?" a narrator says in the ad. "McCain knows Obama denounced Ayers's crimes, committed when Obama was just 8 years old." The ad implicitly suggests Senator McCain was behind the ad attacking Mr. Obama, even though it was bought by an independent group. Mr. McCain has criticized his ties to Mr. Ayers, and his campaign did so again yesterday. "The fact that Barack Obama chose to launch his political career at the home of an unrepentant terrorist raises more questions about Senator Obama's judgment than any TV ad ever could," a spokesman, Brian Rogers, said.