Barack Obama drew massive crowds to some of his final campaign rallies as Americans appeared likely to cap the longest, most expensive White House campaign ever by electing the Democrat as the first black US president on Tuesday. Republican John McCain, looking to score the United States' biggest political upset in 60 years, assured supporters Sunday that the race is tightening. "I've been in a lot of campaigns. I know the momentum is there,'' McCain said at a rally in Pennsylvania, traditionally a Democratic-leaning state that he must wrest from Obama. But polls show Obama leading in Pennsylvania and other key states. Nationally, several major polls show Obama with a 7-8 percentage point advantage. With the economy in turmoil and the approval levels of President George W. Bush, a Republican, at near-record lows, Democrats have high hopes not only of capturing the White House, but also expanding their majorities in both chambers of Congress. A victory would mark a stunning rise for the 47-year-old Obama, who was little known nationally before being elected as a senator from Illinois four years ago. He began running for president just two years later. Obama exuded confidence on Sunday. "The last couple of days, I've been just feeling good,'' he told 80,000 gathered to hear him, and singer Bruce Springsteen, in Cleveland, in the pivotal state of Ohio. "The crowds seem to grow and everybody's got a smile on their face. You start thinking that maybe we might be able to win an election on November 4th.'' An earlier rally in Columbus, Ohio, drew an estimated 60,000 people. Obama has capitalized on anti-Republican sentiment, linking McCain to the unpopular Bush. McCain's campaign has tried to cast Obama as too inexperienced, too liberal and too tainted by associations with unsavory characters. The electoral map clearly favors Obama. To be elected, a candidate must win at least 270 of the 538 electoral votes distributed to states roughly in proportion to their population. In most cases, the candidate who wins a plurality of votes in a state wins all of that state's electoral votes. Obama is favored to win all the states Democrats captured in 2004, when Bush defeated Senator John Kerry. That would give him 251 votes. He is leading or tied in several states won by Bush, giving him several possibilities for reaching the 270 votes, winning a big Bush state like Ohio or Florida, or a combination of smaller ones. Obama campaign manager David Plouffe said Sunday that the Democrat has expanded the electoral map by aggressively campaigning in traditional Republican states like Virginia, Colorado and Nevada. "We did not want to wake up on the morning of Nov. 4 waiting for one state. We wanted a lot of different ways to win this election,'' Plouffe said on Fox television.