Ayers was one of the leaders of the Weather Underground when it was involved in a series of bombings in the 1960s, when Obama was 8 years old. Obama met him in the 1990s when first starting his political career in Chicago and the two served on a board together.

Obama has said he knows Ayers, a professor at the University of Illinois at Chicago, only slightly and has denounced his actions with the Weather Underground.

Republican Sen. Mel Martinez of Florida said on ABC's "This Week" it was not what Obama did when he was 8 but "what occurred when he was 35 - 38 years old and was initiating his political campaign."

"It's about his judgment and who he associated with during those years and right on into his political campaign," he said.

"It is fair game," Sen. Joe Lieberman, an independent from Connecticut who supports McCain, said on "Fox News Sunday."

But Democrats responded that the Republicans were just trying to trivialize the race and take the spotlight off McCain and the economy."

"How ridiculous," Democratic Sen. Claire McCaskill of Missouri said on Fox. "American people deserve so much better."

"You have seen a 26-year Senate veteran morph into an angry, desperate candidate in the last few weeks, especially in the last few days," Democratic Sen. Sherrod Brown of Ohio told ABC. "And it just kind of makes me sad ... that John McCain and Sarah Palin are resorting to these tactics."