US President Barack Obama congratulated Republican rival Mitt Romney on Tuesday for running a hard-fought race for the White House and expressed confidence he would win re-election during a stop at a local campaign office to thank volunteers."I ... want to say to Governor Romney congratulations on a spirited campaign. I know that his supporters are just as engaged and just as enthusiastic and working just as hard today," Obama said as volunteers made phone calls encouraging supporters to get to the polls."We feel confident we've got the votes to win, but it's going to depend ultimately on whether those votes turn out. And so I would encourage everybody on all sides just to make sure that you exercise this precious right that we have that people fought so hard for us to have."Obama made calls to volunteers from the campaign office to thank them for working for his re-election."I expect that we'll have a good night, but no matter what happens, I just want to say how much I appreciate everybody who supported me, everybody who's worked so hard on my behalf," he said.Opinion polls show Obama and Romney in a virtual dead heat, although the Democratic incumbent has a slight advantage in several vital swing states that could give him the 270 electoral votes needed to win the state-by-state contest.Traditionally presidential candidates get media attention on Election Day by going to vote. But Obama cast his ballot in Chicago last month - part of his campaign's push to get its supporters to vote early. So the president's visit to the office gave him a chance to get in front of the cameras, generate news coverage and encourage turnout.Obama and Romney, a former Massachusetts governor and private equity executive, have fought a largely negative campaign. Obama's team attacked Romney for his business record, and Romney's team criticized the US president for presiding over high unemployment and a slow economic recovery.Obama's conciliatory comments represent the close of the bitter campaign and could appeal to last-minute undecided voters, who are turned off by the lack of bipartisanship in Washington.Campaign teams continue to work at 12th hourAs Americans headed to voting booths, campaign teams for both candidates worked feverishly at the last minute to mobilize supporters to cast their ballots.Polls will begin to close in Indiana and Kentucky at 6 p.m. EST on Tuesday, with voting ending across the country over the next six hours.Romney voted at a community center near his home in a Boston suburb before dashing off for two last-minute stops, including the most critical of all swing states - Ohio. "I feel great about Ohio," he said when asked about what is considered a must-win for him.The close presidential race raises fears of a disputed outcome similar to the 2000 election, which was decided by the US Supreme Court. Both campaigns have assembled legal teams to deal with possible voting problems, challenges or recounts.The balance of power in the US Congress will also be at stake in Senate and House of Representatives races that could impact the outcome of "fiscal cliff" negotiations on spending cuts and tax increases, which kick in at the end of the year unless a deal is reached.Obama's Democrats are now expected to narrowly hold their Senate majority, while Romney's Republicans are favored to retain House control.The presidential contest is now likely to be determined by voter turnout - specifically, what combination of Republicans, Democrats, white, minority, young, old and independent voters shows up at polling stations.Obama and Romney raced through seven battleground states on the final day of campaigning to hammer home their final themes, urge supporters to get to the polls and woo the last remaining undecided voters.Obama focused on Wisconsin, Ohio and Iowa, the three Midwestern swing states that, barring surprises elsewhere, would give him 270 electoral votes. Romney visited the must-win states of Florida, Virginia and Ohio before finishing in New Hampshire, where he launched his presidential run in June 2011.After two days of nearly round-the-clock travel, Obama wrapped up his final campaign tour in Des Moines, Iowa. Romney's final day included stops in Florida, Virginia, Ohio and New Hampshire.The common denominator for both candidates was Ohio, the most critical of the battlegrounds, particularly for Romney. Without the state's 18 electoral votes, the path to victory becomes very narrow for the Republican.