Obama, who held his ground in the third and final debate, is headed to Virginia and Missouri later this week - states that often have been out of reach for Democrats in past elections but are up for grabs this year.
Looking to shake up the race, McCain questioned Obama's character and his policies Wednesday night. He linked Obama to a 1960s radical, accused him of planning tax increases that would cripple the economy and said he was dishonest about a promise to accept public campaign financing.
"You didn't tell the American people the truth," the Arizona senator said.
Obama ignored that charge and remained calm throughout the debate. He often turned the accusations back against McCain, calling them examples of the petty politics harming the country.
"The important point here is, though, the American people have become so cynical about our politics, because all they see is a tit-for-tat and back-and-forth," the Illinois senator said. "And what they want is the ability to just focus on some really big challenges that we face right now."
McCain went on offense from the opening moments of the debate, accusing Obama of waging class warfare by seeking tax increases that would "spread the wealth around."
He 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. McCain said the group could be on the verge of "destroying the fabric of democracy."
McCain sought to blunt one of Obama's sharpest lines of attack: the Republican's ties to the unpopular incumbent president. "I am not President Bush. If you wanted to run against President Bush, you should have run four years ago," McCain said.
Obama returned each volley, and brushed aside McCain's claim to full political independence.
"If I've occasionally mistaken your policies for George Bush's policies, it's because on the core economic issues that matter to the American people - on tax policy, on energy policy, on spending priorities - you have been a vigorous supporter of President Bush," he said.
He condemned Ayers' violent activities and denied any significant ties to ACORN, mocking McCain for bringing them up.
"I think the fact that this has become such an important part of your campaign, Sen. McCain, says more about your campaign than it says about me," Obama said.
The 90-minute encounter at Hofstra University marked the beginning of a 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.
Associated Press Writer Beth Fouhy contributed to this report.