While the overall number of early votes was unknown, statistics showed more than 29 million ballots cast in 30 states and suggested an advantage for Obama. Democrats voted in larger numbers than Republicans in North Carolina, Colorado, Florida and Iowa, all of which went for President Bush in 2004.
Among the two presidential contenders, Obama was the first to cast his ballot Tuesday.
The Illinois senator, accompanied by his wife, Michelle, and their two daughters, Malia, 10, and Sasha, 7, voted early at Chicago's Beulah Shoesmith Elementary School. The girls looked on as the Obamas, standing in side-by-side booths, marked their long paper ballots.
"I feel great and it was fun, I had a chance to vote with my daughters," Obama told reporters afterward. "I feel really good."
Asked if he was feeling sentimental, Obama replied: "You know I'm sure I will tonight —that's when polls close."
"The journey ends but voting with my daughters, that was a big deal," he added. "I noticed that Michelle took a long time though. I had to check to see who she was voting for."
OBAMA: Candidate casts ballot
Obama planned to go to neighboring Indiana for a final campaign appearance then attend a huge outdoor rally in Grant Park in Chicago.
His Republican rival was scheduled vote later in the day in Arizona then hold final rallies in Colorado and New Mexico and return for a party at the Biltmore Hotel in Phoenix.
In House and Senate elections, Democrats seemed poised to extend their hold on Congress.The GOP was fighting a strong headwind due to the economic crisis and an unpopular incumbent Republican president.
In addition, of the 35 Senate races on Tuesday's ballot, 22 are now held by Republicans, 13 by Democrats. Senate Democrats hold a narrow 51-49 majority, and only thanks to the support of two independents.
Their goal Tuesday is reaching a coveted 60-seat, filibuster-proof Senate majority. Leaders in both parties portrayed that as a long shot. Sen. John Ensign, R-Nev., head of the party's senatorial campaign committee, acknowledged ahead of the voting that "Democrats are poised to pick up some seats."
His Democratic counterpart, Sen. Charles Schumer, D-N.Y., predicted "a whole lot of seats" for Democrats, but said reaching a 60-vote majority was unlikely.
In the House, Democrats were counting on heavy turnouts to capture more than 15 GOP seats, and they had a good chance to wrest away another two dozen seats. Republicans had fewer than a dozen Democratic targets they had any hope of defeating.
If so, it would be the first time in more than 75 years that Democrats would ride large waves of victory to bigger congressional margins in back-to-back elections.
For both presidential candidates, Election Day was relatively serene after Monday's full-day of multi-state campaigning that began in Florida.
"I'm feeling kind of fired up. I'm feeling like I'm ready to go," Obama told nearly 100,000 people gathered for his final rally Monday night in Virginia.
"At this defining moment in history, Virginia, you can give this country the change it needs," Obama said to voters in a state that hasn't voted for a Democratic presidential nominee in 44 years.
On the plane back to Chicago, Obama strolled back to speak with reporters who have been covering his campaign, saying he wanted to shank them for having "shared this process with us."
"Ok guys, let's go home," he told the reporters. "It will be fun to see how the story ends."
McCain closed out his election-eve sprint with a return to Prescott, Ariz., where he has traditionally ended his campaigns.
MCCAIN: Final sprint through battleground states
"This momentum, this enthusiasm convinces me we're going to win tomorrow," McCain told a raucous evening rally in Henderson, Nev., in one of his last stops.
Speaking Tuesday on ABC's "Good Morning America," the Arizona senator said he had no regrets about his campaign.
"It's been one of the most incredible experiences that anyone can have," McCain. "I've loved every minute of it. Every day, being able to meet the people we've met and go the places we've gone, it's been an unforgettable experience."
He told CBS's Early Show that the believes that most battleground states have "closed up" and that there is a "a good scenario" for a victory.
Obama's running mate, Sen. Joe Biden, voted at mid-morning in Wilmington. McCain's running mate, Alaska Gov. Sarah Palin, returned to Alaska to vote later in her hometown of Wasilla.