Sarah Palin, McCain's running mate, arrived in Anchorage, Alaska, overnight and voted in her hometown of Wasilla. She is expected to fly to Phoenix to join McCain in time to watch the results come in.
After voting, Palin noted that the result would be historic, implying that the voters would elect either the country's first black president or the first female vice president.
"It bodes so well the progress our country is making," Palin said.
She also said she was delighted that she was cleared of any wrongdoing in the firing Alaska's top police officer.
"You didn't believe us," she told reporters. "I told you we'd done nothing wrong."
In Florida, a swing state marginally leaning Republican, voters in some cities were confronted with long lines, while others found shorter queues in areas that had high early voter turnout. By some estimates nearly 50 percent of Florida's likely voters had cast absentee or early ballots.
"If Obama gets elected, I'll definitely go back [to Iraq] because they are going to test him," said a 40-year-old Air Force colonel who voted Republican in Sarasota, Fla., and asked that his name not be used. "Obama's done nothing except get endorsed by Oprah. McCain will bring troops home in an orderly fashion."
Today's vote caps a long-fought and record-breaking campaign between two candidates who were both written off early in their candidacies and whose races for the White House have been nothing short of history making.
"The election is historic by any standard. Barack Obama might become the first African-American president. One woman, Hillary Clinton ran and nearly secured her party's nomination; another, Sarah Palin, could potentially be the first female vice president," said Matthew Dowd, an ABC News political consultant. "We're seeing a great generational shift."
By almost every quantifiable measure -- from the $640 million Obama raised in the month of October, to the nearly $1 billion combined the campaigns have spent, to 9 million newly registered voters -- records have been shattered.
Yet another record may fall before the day is over as turnout is heavy and could surpass previous voting turnout records. The existing turnout records were set in 2004 when more than 122 million Americans went to the polls, and in 1960 when 64 percent of eligible voters cast a ballot.
All eyes are now on a handful of battleground states that can turn the election or signal a sea change in American politics.
While McCain faces a tough road to the necessary 270 electoral votes, victories in tightly contested Florida, Ohio and Virginia could significantly shift the electoral map in his favor and result a win.
Polls close earliest in Virginia -- at 7 p.m. ET -- and in Indiana, some polls close at 6 p.m. ET (that's 7 p.m. ET).
"The first thing to look at is Virginia," said Dowd. "If Obama has the lead there, you know the outcome of the race. If McCain wins or it's too close to call, anything can happen."
The election comes in the midst of the worst economic crisis since the Great Depression, and at a time when 85 percent of the populace believes America is on the wrong track.