"There are two full days to go before Election Day and obviously anything can happen, but it is hard to see where McCain goes from here," said pollster John Zogby.
Obama warned against overconfidence at a rally in Columbus, Ohio attended by more than 60,000 people.
"Don't believe for a second that this election is over," said Obama, who if elected would be the first black US president.
Al Jazeera's Rob Reynolds in Ohio said Obama and Joe Biden, his running-mate, had targeted the state in an attempt to attract voters who had supported Hillary Clinton, Obama's rival during the long Democratic primary battle.
On Sunday, the 72-year-old Arizona senator again attacked his opponent's patriotism and tax plans and insisted he remained on course for the White House.
"I've been in a lot of campaigns, I know when momentum is there. We're going to win Pennsylvania, we're going to win this election," McCain said in Pennsylvania.
Obama attacked McCain after Dick Cheney, the vice-president, hailed the Arizona senator as the right man to lead the country because he "understands the danger facing America".
The Democratic campaign aired a new television advertisement highlighting McCain's endorsement by Cheney, who has been linked to many of the most controversial decisions made by the unpopular Bush administration.
The advertisment contrasted the endorsement with the backing Obama has won from Colin Powell, the former Republican secretary of state, and Warren Buffett, the billionaire investor.
The presidential campaign has narrowed down to states that have been reliably Republican in recent elections or, in the case of Virginia, Indiana and North Carolina, that have not voted for a Democratic hopeful in decades.
Rob Reynolds said Obama was also targeting traditionally Republican states in the hope of taking some key electoral college votes in areas that were not traditional swing states, such as Ohio and Florida.
Under the US political system, the president is elected not by direct popular vote but by capturing 270 out of 538 electoral votes distributed throughout the country in a state-by-state contest.