Mitt Romney notched early wins as he fought to establish his dominance in the race for the Republican US presidential nomination but Ohio, the biggest prize of the evening, was too close to call.
Romney won as expected in Massachusetts, Virginia and Vermont, while rival Newt Gingrich won his home state of Georgia, according to TV network projections. Rick Santorum won in Tennessee and Oklahoma.
Results from the other states holding contests on "Super Tuesday," the biggest day so far in the roller coaster Republican campaign, were expected in the coming hours. Some 419 of the 1,144 delegates needed to win the party's nomination are at stake.
Polls have closed in seven of 10 states in the largest day of state-by-state voting yet in the 2012 election cycle.
Four candidates are still competing to become the conservative party's choice in the November elections against President Barack Obama, who is unopposed for renomination in the Democratic Party.
States as diverse as Georgia in the South, the industrial Midwestern state of Ohio and sparsely populated Alaska were casting ballots. The last votes will be cast in Alaskan caucuses, which close at 8 pm (0500 GMT Wednesday).
Romney hopes to extend his lead as Republican frontrunner, but apart from Gingrich also faces former Pennsylvania senator Rick Santorum and Texas Congressman Ron Paul.
A strong showing on Super Tuesday could provide the strategic boost that Romney needs to claim the party mantle.
The sheer number of states in play, with their diverse demographics and political leanings, could break open a candidate's pathway to the nomination, or spell the end of a failing campaign.
Super Tuesday essentially ended the Republican nomination battle in 2008, when Romney capitulated and Senator John McCain went on to become the nominee.
'Good luck tonight'
President Barack Obama even weighed in on the contest earlier in the day at a White House press conference.
"What would you like to say to Mr Romney?" Obama was asked during hours before polls began to close.
"Good luck tonight," he said with a grin, eliciting laughter from reporters.
Ohio is seen as the big prize on Super Tuesday, because it is key in November presidential elections - no Republican has ever been elected president without winning there - and its demographic mix closely mirrors the nation at large. The race there has been too close to call between Romney and Santorum in recent opinion surveys.
The Super Tuesday vote could give momentum to other candidates. Santorum hopes for a strong showing in Southern states, including Tennessee, where evangelical Christian voters make up a larger share of the electorate and social issues play a larger role.
All eyes on Ohio
With 437 delegates to August’s Republican National Convention at stake - nearly 40 per cent of the 1,144 needed to secure the nomination - Romney and Santorum have been criss-crossing the country in a frenzy of campaigning.
Romney got a boost going into Tuesday's votes with a win in the Washington state caucuses, but that contest was non-binding.
Romney, Santorum and Gingrich all spent Monday campaigning in Ohio, which for decades has served as a "bellwether" for the general election, voting for the eventual winner.
With the back-and-forth competition still undecided, some are starting to speculate about a possible compromise convention when the Republican Party gathers to select their candidate later this year.
If no one is able to win the 1,144 delegates needed, then the floor could open to potential candidates not currently in the race.
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