John McCain repeatedly 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 less than three weeks from Election Day.

"It''s not true," McCain retorted.

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

McCain is currently running all negative ads, according to a study by the University of Wisconsin-Madison. But he has run a number of positive ads during the campaign.

The 90-minute encounter, seated 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.