The Democrats are reaching for a 60-vote Senate majority that would allow the party to overcome Republican filibusters, and could pick up two dozen or more House seats. Democrats also hope to pad their slim majority of governorships and increase their ranks in what already is their strongest majority in state legislatures in more than a decade.

The implications are far-reaching: Governors and state legislators elected Tuesday to four-year terms will help preside over the redrawing of legislative and congressional districts following the 2010 Census. The party in charge can redraw districts in its favor.

Atop the ticket, Obama leads in national and key battleground state polling, though the race appears to be tightening as it plays out primarily in states that Bush won twice. Among the unknowns: the choices of one in seven likely voters who are undecided or could still change their minds; the impact of Obama's efforts to register and woo new voters, particularly blacks and young people; the effect of Obama's race on voters just four decades after the tumult of the Civil Rights movement.

"Right now, it's very clearly Obama's to lose, and I think his chances of doing so are pretty minimal," said Republican Dick Armey, the former House majority leader from Texas. He said the possibility of a McCain comeback is "getting down to slim-to-none."

An Obama victory would amount to a wholesale rejection of the status quo: voters taking a chance on a relative newcomer to the national stage, a 47-year-old first-term senator from Chicago, rather than stick with a seasoned veteran of the party in power. With strengthened Democratic majorities in Congress, he'd have to deal with the party's left flank while governing a country that's more conservative than liberal.

The Republican Party essentially would be in tatters, searching for both a leader and an identity.

An Obama loss — or McCain comeback — would be a crushing disappointment for Democrats in a year tailor-made for the party. It would suggest McCain's experience trumped Obama's clarion call for change, and raise troubling questions about white Americans' willingness to vote for a black man.

Blacks, in particular, might be furious and deeply suspicious of an almost sure thing that slipped away.

Tuesday's election caps a nearly two-year campaign unprecedented in many ways, merely unusual in others.

"The candidates are more interesting. The media is bigger. The technology is better. Participation has increased dramatically," said Bob Kerrey, a former Democratic senator from Nebraska who once aspired to the presidency himself. "This is the first global campaign that the United States has had. People will always remember this as an extremely important election."

From the start, the race was different: It was the first since 1928 in which neither a president nor a vice president competed.