Polls closed in six states at 7 p.m. ET Tuesday, and Fox News projects Mitt Romney carrying Indiana and Kentucky, with President Obama winning Vermont.
Three other states, though -- among them the key battleground of Virginia -- were too close to call. South Carolina and Georgia are the other two.
In Virginia, Fox News exit polls show the race so tight that neither Romney nor Obama have even a slight lead. Ohio, another crucial battleground, will close its polls at 7:30 p.m. ET.
While Romney is now ahead in the electoral-vote count, the lead will likely swing back and forth as more returns come in.
The razor-thin margin between the candidates means the campaigns could be in for a long night, after the candidates wound down their campaigns earlier in the day.
Election Day, though, was unexpectedly busy for the campaigns. While Obama himself kept a low profile in Chicago, the campaign dispatched Vice President Biden to Ohio where he visited a Cleveland restaurant and later posed for pictures with volunteers before joining up with the president.
Romney, meanwhile, made stops in Ohio and Pennsylvania -- two key swing states -- before heading back to campaign headquarters in Boston.
"This is a big day for big change," Romney told a crowd of campaign volunteers outside Cleveland.
The visits rounded out a grueling battle for the White House. For Obama, the election is the last time he says his name will appear on a ballot. For Romney, the vote marks the close of a nearly six-year run for the presidency.
In an airtight contest, both candidates were expressing confidence as millions of voters flocked to the polls. Obama visited a Chicago campaign field office Tuesday morning, before playing his traditional Election Day game of pick-up basketball.
"The great thing about these campaigns is, after all the TV ads and all the fundraising and all the debates and all the electioneering, it comes down to this," Obama said.
Voting kicked off overnight in two tiny villages in northern New Hampshire. Obama and Romney each won five votes in the small town of Dixville Notch, which was the first to announce its results after polls opened and closed within 43 seconds.
In Hart's Location, N.H., Obama won with 23 votes, Romney received 9 and Libertarian Gary Johnson received 1 vote after 5 minutes, 42 seconds of voting. The towns have enjoyed first-vote status since 1948.
Obama closed out his campaign with a late-night rally in Iowa on Monday. The event was held in the same state where his 2008 caucus victory jump-started his path to the White House.
The president was photographed with tears running down his face as he spoke before a crowd of 20,000 supporters, telling them "this is where our movement for change began."
The president never mentioned Romney in his closing appeal, which sought to draw on the hope and optimism of his first campaign.
Romney returned Monday night to New Hampshire, where he launched his bid, telling supporters during his final campaign rally that he needs their votes again.
"It's all your votes and your work right here in New Hampshire that will help me become the next president of the United States," Romney told a thunderous capacity crowd at the Verizon Wireless Arena, which holds about 10,000 people. "We thank you and we ask you to stay in it all the way to the victory tomorrow night."
The final hours of the exhausting 2012 contest were played out at earlier mega-rallies across a half-dozen states. Each candidate sought to close the deal with voters promising "change," while accusing the other of peddling an agenda that would choke the country's already meager economic recovery.
Ohio is arguably the hardest-fought contest of the race, with both candidates visiting Monday and both pouring millions into that battleground. But with polls giving Obama a slight edge in the Buckeye State, Romney's campaign also has made a late play for Pennsylvania -- a win in Pennsylvania could allow the Republican nominee to lose Ohio and still have a pathway to the 270 electoral votes it takes to win.
The Obama campaign, though, dismissed Romney's Election Day efforts as a "Hail Mary."
Voters across the country on Tuesday were casting ballots not just in the presidential race, but a host of congressional races that will determine the balance of power on Capitol Hill next year. Democrats currently control the Senate by a narrow 53-47 margin.
It's still an uphill climb for Republicans to take control -- they have a much easier shot at retaining control of the House.
Meanwhile, voters are deciding on an array of controversial measures at the state level, including several that would partially legalize marijuana.
In the presidential race, battleground polls show a mixed picture. While Obama has the edge in Ohio, for example, Romney has the edge in the crucial state of Florida. Nationally, the latest and final Gallup survey showed Romney with 49 percent and Obama with 48 percent support.
By most estimates, Obama comes into Election Day with a slight advantage in the electoral vote count. The RealClearPolitics electoral map shows the states likely to vote for Obama are worth 201 electoral votes, while those likely to vote for Romney are worth 191. It takes 270 to win.
The toss-ups include: Colorado, Florida, Iowa, Michigan, Nevada, New Hampshire, North Carolina, Ohio, Pennsylvania, Virginia and Wisconsin.