Mr. Obama and Romney spent those final hours making their last appeals to voters in the crucial swing states. President Obama stopped in Wisconsin, Iowa and Ohio before heading to Chicago for election night. Romney hit Florida, Virginia, Ohio and New Hampshire before returning to election headquarters in Boston.
Ohio is a key battleground state. Whoever takes that state will have an easier road to the presidency. National polls early Monday showed that race is at a virtual tie.
Mr. Obama, Romney and allied groups have spent more than $1 billion on television advertising since June, primarily in Colorado, Florida, Iowa, New Hampshire, Nevada, North Carolina, Virginia and Wisconsin.
Both candidates pulled out all the stops. Mr. Obama was backed with star power over the last few days, including Monday when rocker Bruce Springsteen performed for the president's supporters, a crowd of about 18,000, in Madison, Wis. Rap heavyweight Jay-Z warmed up the commander in chief's crowd in Columbus, Ohio.
Mr. Obama's face was plastered everywhere and just about everything in Chicago in 2008. But there was a far different energy in the city's streets Monday.
"Over the four years, people have become a little more divided in terms of support," said Chicago resident Patrick Wimp. "So I think there's not as much of that energy."
But others don't see the absence of obvious support as a negative thing.
"I don't think there's any less support than there was four years ago, it's just a little toned down," said Joe Greskoviak.
The president planned to spend Tuesday with family and friends in Chicago.
Meanwhile, Romney kicked off his day in Florida - one of the biggest prizes with 29 electoral votes. He told supporters that a better economy was just 24 hours away.
"That's why I'm running for president: I know how to change the nation, how to get it back on-course, how to create jobs, how to get a balanced budget, how to get rise in take-home pay," Romney said amid cheering supporters and flurry of flapping red, white and blue Romney-Ryan signs. "Accomplishing real change is not something I just talk about. It's something I've done, and it's something I'm going to do when I'm president of the United States of America.".
Romney finished campaigning Monday in New Hampshire, which is worth four electoral votes. But in an election so close, those votes could make all the difference.
Kyle Downey, a spokesman for the Romney campaign, said they are confident they will win some states once thought safe for Mr. Obama.
"We have a very advanced ground game," Downey said. "We have 34,000 volunteers across the country who are going to help us get folks out and track the vote. We feel very good about how folks will turn out tomorrow."
Romney planned to continue campaigning Tuesday. He will be in Cleveland, Ohio, and Pittsburgh, Pa.
More than 27,000,000 Americans have already cast their votes in 34 states and the District of Columbia. Voters in Los Angeles were allowed to vote early Monday at the Los Angeles County Registrar's Office in Norwalk from 8 a.m. to 5 p.m.
There were already problems at Florida polls over the weekend. Several polling places in Palm Beach, Miami-Dade and Broward counties had lines with wait times of five to six hours on Saturday. Democrats filed a partially-successful lawsuit Sunday in an effort to extend voting hours in those places. A judge granted their request in one county where an early voting site was shut down for several hours Saturday because of a bomb scare.
Four years ago there was a similar situation and the governor allowed for extended hours.
Look for ongoing reports by Elex Michaelson from the Obama campaign headquarters in Chicago and Carlos Granda from the Romney campaign headquarters in Boston through the end of the presidential election.
Tune in to ABC7 on Tuesday for complete Election Day coverage:
3 p.m. to 3:30 p.m.: Special Edition of Eyewitness News
3:30 p.m. to 4 p.m.: World News with Diane Sawyer
4 p.m. to 11 p.m.: ABC News Election Coverage
11 p.m.: Eyewitness News
Live coverage on abc7.com begins at 4 p.m.
The Associated Press contributed to this report.