With the pivotal Pennsylvania primary less than a week away, the Democratic presidential contenders went for the knockout in Wednesday night's debate.Senator Hillary Clinton tried to put her opponent, Barack Obama, on the defensive by linking him to a former '60s radical. Wednesday night's debate on ABC attracted nearly 11 million viewers, twice as many as any of the previous 20 Democratic confrontations. And it featured a lot of questions about verbal gaffes and controversial relationships, which in itself is sparking a lot of criticism of ABC. And to the delight of the Clinton campaign, most of the heat was on Obama, including questions about his relationship with the leader of a violent 1960's radical group. The Clinton campaign is trying to keep Democratic frontrunner Obama back on his heels Thursday by pressing an issue raised for the first time in Wednesday night's debate in Philadelphia. And that is Obama's friendship and service on a civic board with his Hyde Park neighbor, Bill Ayers, a UIC professor who led the Violent Weather underground radical group that bombed government and military facilities in the '60s and '70s. "I hope that he will be in a position to explain how it was that William Ayers hosted a political event for him at his home," said Clinton campaign manager Howard Wolfson. Obama bristled at the guilt-by-association implication when the Ayers connection came up Wednesday night. "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," said Obama. "They were turbulent, difficult times in this country, very difficult times, many times in families," said Chicago Mayor Richard Daley. On Thursday, Mayor Daley was defending Obama, pointing out that Ayers comes from a prominent, civic-minded Chicago family and that Ayers himself has worked tirelessly on school reform for several decades. "This has nothing to do with Sen. Obama. I am a friend of Bill Ayers and his family and proud to be that," said Daley. Obama was not talking specifically about Bill Ayers as he campaigned in North Carolina Thursday. But he is ridiculing a debate that went on for nearly an hour without a question about a bread-and-butter issue that affects voters directly. "It took us 45 minutes before we even started talking about a single issue that matters to the American people," said Obama. Obama says that Clinton and her campaign love the gotcha game because they played it for eight years when Bill was in the White House, it's all they know, and if he's elected that will change. But the Clinton campaign says if Obama can't take the heat now on any and all questions, how can he beat John McCain and the Republicans in the fall. Ayers was unavailable for comment. He's out of town in Georgia where he was scheduled to give a speech Thursday morning but cancelled at the last minute. The Obama campaign says that for the record Ayres never held a fundraiser for any of Obama's campaigns.