HEMPSTEAD, N.Y. – John McCain assailed Barack Obama’s character and campaign positions on taxes, abortion and more Wednesday night, hoping to transform their final presidential debate into a launching pad for a political comeback. “You didn’t tell the American people the truth,” he charged.

Unruffled, and ahead in the polls, Obama parried each accusation, and leveled a few of his own.

“One hundred percent, John, of your ads, 100 percent of them have been negative,” Obama shot back in an uncommonly personal debate.

“It’s not true,” McCain retorted.

“It absolutely is true,” said Obama, seeking the last word.

The 90-minute encounter, at a round table at Hofstra University, was their third debate, and marked the beginning of a 20-day sprint to Election Day. Obama leads in the national polls and in surveys in many battleground states, an advantage built in the weeks since the nation stumbled into the greatest economic crisis since the Great Depression.

With few exceptions, the campaign is being waged in states that voted Republican in 2004 – Virginia, Colorado, Iowa – and in many of them, Obama holds a lead in the polls.

McCain played the aggressor from the opening moments of the debate, accusing Obama of waging class warfare by seeking tax increases that would “spread the wealth around.”

The Arizona senator also demanded to know the full extent of Obama’s relationship with William Ayers, a 1960s-era terrorist and the Democrat’s ties with ACORN, a liberal group accused of violating federal law as it seeks to register voters. And he insisted Obama disavow last week’s remarks by Rep. John Lewis, a Democrat, who accused the Republican ticket of playing racial politics along the same lines as segregationists of the past.

Struggling to escape the political drag of an unpopular Republican incumbent, McCain also said, “Sen. Obama, I am not President Bush. … You wanted to run against President Bush, you should have run four years ago.”

Obama returned each volley, and brushed aside McCain’s claim to full political independence.