Polls have opened in Ohio as the primary and caucus contests in 10 US states gets under way, heating up the battle for the Republican Party’s presidential nomination.
Alaska, Georgia, Idaho, Massachusetts, North Dakota, Oklahoma, Tennessee, Vermont and Virginia will also have their say on a day dubbed "Super Tuesday" that has traditionally been decisive in determining who goes on to claim the nomination.
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TV networks projected Newt Gingrich as the winner in his home state of Georgia, giving the former congressman his second victory of the primary season.
Gingrich, who spent much of the last week campaigning on his home turf, last won a victory in January in South Carolina.
Georgia has the biggest number of delegates of the states holding nominating contests on Tuesday and Gingrich had said he had to win the state to keep his campaign viable.
Frontrunner Mitt Romney hoped to keep momentum on his side and deliver a major blow to chief rival Rick Santorum as the two candidates waged a closely fought battle over Ohio, seen as the biggest prize of the day.
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.
Four years later, Romney is in the lead and could possibly seal the nomination on Tuesday. However, Santorum is close behind, still hoping to pull off a stunning upset by drawing on growing support among the Republicans' conservative base. Newt Gingrich and Ron Paul are also still in the race.
But many analysts predict that no clear winner will emerge.
"I'm not sure Tuesday means that much other than more of the same," said veteran strategist Neil Oxman, co-founder of The Campaign Group.
All eyes on Ohio
"What's going to happen is, Santorum is going to do well enough to continue," while Romney will emerge from Super Tuesday as "the favourite, but not the overwhelming favourite," Oxman told AFP news agency.
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.
One important factor is Gingrich, who is hoping to inject some new momentum into a flagging campaign. He is polling ahead in Georgia, the state he served in congress for 20 years, and which has Tuesday's largest haul of 76 delegates.
Gingrich has been vying with Santorum to be the conservative alternative to Romney, but has faltered since a January win in South Carolina.
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.
Al Jazeera’s Alan Fisher in Washington blogged about possible outsiders who could come into the race. Two popular Republican politicians, former Alaska governor Sarah Palin, who ran alongside McCain in 2008, and former Florida governor Jeb Bush, the younger brother of former president George Bush, are two potential candidates.
“Though anyone who predicts what can happen in this latest campaign may be misguided, the fact that Republicans still hanker for someone else shows deep dissatisfaction with the choice they have before them,” Fisher said.
“But at this late stage, it’s likely to be the only choice they’ll have.”