In an effort to expand the electoral map, Romney has made a late run at Pennsylvania, where recent polls have shown him within striking distance of the president. If Romney can secure 20 electoral votes from the Keystone State, which Mr. Obama won by ten percentage points four years ago, it would dramatically expand his potential paths to victory.

David Axelrod, senior adviser for the Obama campaign, told CBS News Monday that while he expects the race to be close, "we have a good chance to break 300" electoral votes. Despite a narrower path to victory, the Romney camp is also exuding confidence: It has pointed to polls suggesting greater voter enthusiasm among Republicans to argue that a robust turnout among Romney supporters will allow the former Massachusetts governor to cruise to a clear victory.

Romney voted in Belmont, Mass., on Tuesday morning before his trip to Ohio and Pennsylvania; he will return to Massachusetts to watch the returns with staffers and supporters in Boston. Mr. Obama, who voted early in an effort to encourage supporters to do the same, will watch the returns from his campaign headquarters in Chicago. The vice presidential nominees, Joe Biden and Paul Ryan, voted Tuesday morning.

Even before Election Day got underway, problems were emerging for some Americans seeking to cast their ballots. In New York and New Jersey, election officials were scrambling to accommodate voters in the wake of Superstorm Sandy; New Jersey voters were given the option to vote by email or fax, while in New York officials are allowing affected voters to cast ballots at any polling place. In Miami on Sunday, election officials temporarily closed their doors to voters who stood in long lines in an effort to cast early absentee ballots amid a bitter fight between the state Democratic and Republican parties over early voting. In Ohio - where a tight result could mean weeks of battles over provisional and absentee ballots and other issues - Republican Secretary of State Jon Husted is facing judges over a directive that could mean the invalidation of certain provisional ballots.

The presidential race is not the only contest taking place Tuesday, of course. Americans will also cast ballots in Senate, House and gubernatorial races and decide ballot measures on issues including taxes, same-sex marriage, the health care law and the legalization of marijuana. Republicans are expected to maintain control of the House, though Democrats could make gains; Democrats are expected to maintain their advantage in the Senate, though there are enough close races that Republicans are holding out hope for a takeover.