COLUMBUS: Voters from Alaska to Georgia are going to the polls on a potentially pivotal Super Tuesday that is expected to see Mitt Romney close in on the Republican presidential nomination.

The frontrunner hopes to build on recent momentum, he has won the last five voting contests, and deal a knockout blow to key rival Rick Santorum as they fight for the right to take on President Barack Obama in November.

In Tuesday's 10-state marathon, Romney is tipped to win Idaho, Massachusetts, Vermont and Virginia, Santorum is odds-on to take Oklahoma and former House speaker Newt Gingrich is seen as a sure-fire winner in his home state of Georgia.

Ohio and North Dakota are straight shoot-outs between Romney and Santorum, while all three have a chance in Tennessee. Romney must fend off a strong challenge in Alaska from Texas congressman Ron Paul, who is still looking for his first state win.

Romney will not be crowned the nominee on Tuesday and there could still be a long battle all the way to the Republican Party's August convention, but if he wins the crucial state of Ohio he will become very hard to beat.

Exiting a TV studio in Columbus, Ohio after addressing the pro-Israeli AIPAC lobby in Washington via video-link, Romney told AFP: “It's a great day, looking forward to it.”

Asked if he would win Ohio, he replied: “I hope so.”The former Massachusetts governor gave the thumbs up before boarding his campaign bus, which has been frantically criss-crossing this Rust Belt state as he tries to overcome a stiff challenge from Santorum.

Polls opened at 6:30 am (1130 GMT) in the largely working-class swing state and general election bellwether.

A steady stream of voters showed up before work at a polling station in the Whetstone Community Center in an affluent area of Columbus.Jim Ray, 46, who runs a truck dealership, said he backed Romney.

“I just think he has the best chance of winning against Obama. My biggest concern is who will be able to win in November,” he told AFP.

Romney talked up Ohio's importance on Monday, characterizing the race as a “battle for the soul of America” and telling cheering supporters at a rally in Zanesville: “If you do your job tomorrow we're going to win this thing.”

Santorum, a devout Roman Catholic who fiercely opposes abortion and gay marriage, has billed himself as an authentic conservative who understands working-class voters and can beat Obama in the manufacturing-heavy Rust Belt.

If he loses Ohio, serious questions will be asked about his electability in November. Final polls showed the race was too close to call but Romney has closed fast after Santorum enjoyed a healthy lead a week ago.

Santorum, who came from nowhere to sweep three states on February 8, challenging Romney for frontrunner status before faltering in Arizona and Michigan, has played up his underdog status.

“The establishment is lining up behind the guy who's next in line,”Santorum told supporters Monday in Westerville, Ohio. “We have to have someone who can stand on principle, stand on ideas.”

Delegates are awarded by each state in the complex Republican Party nominating process, sometimes on a proportional and/or non-binding basis, until one candidate reaches the 1,144 delegate threshold required to declare victory.

Romney, a former businessman who made his fortune as a venture capitalist, has pulled ahead in the early delegate count, but Santorum and Gingrich loom in the wings as dark horses for the nomination.