Mitt Romney can take a big step toward finishing off chief rival Rick Santorum and seizing command of the Republican presidential race on Tuesday as 10 states hold contests with Ohio at ground zero.

Romney, already the winner of the last five contests, carried momentum into Super Tuesday. Polls showed he had erased a double-digit lead to Santorum in Ohio and is now running roughly even with him.

It is the biggest day so far in the Republican race to find a challenger to President Barack Obama in the November 6 election. At stake are 419 of the 1,144 delegates needed to win the party's nomination.

Ohio is the largest battleground and most closely watched prize of the 10 contests on Super Tuesday. But other critical contests are being held. Former House of Representatives Speaker Newt Gingrich, looking for a path to a comeback, leads his home state of Georgia.

Santorum is ahead in Oklahoma and libertarian Congressman Ron Paul hopes to score his first win in Alaska.

Romney is favoured in Virginia and Vermont and his home state of Massachusetts, and hoped to score an upset in Tennessee, where he was competing strongly with Santorum.

A victory in Ohio and a good showing elsewhere would make Romney the favourite to win the nomination after a grinding, months-long battle in which he has been challenged by a series of conservative alternatives.

If there has been any consistency to the 2012 Republican race, however, it has been its unpredictable nature. There are still many contests to come and many conservatives still harbour serious doubts about Romney.

Santorum, a former Pennsylvania senator with a populist streak and a penchant for wearing sweater vests, is sounding like a candidate who realizes he must win Ohio to retain credibility as the main Romney rival. He has been vastly outspent in Ohio by a Romney machine that has pelted him with negative ads.

A win for Santorum here would turn the race upside down again.

It's gut check time, Santorum told supporters in Cuyahoga Falls on Monday night. Who wants it the most? ... I'm a fighter and a scrapper.

He sought to raise doubts about Romney in the final hours by focusing on the healthcare plan Romney developed as governor of Massachusetts and which Democrats say was the model for the overhaul adopted by Obama. Republicans, Romney included, want to repeal it.

Santorum said the fact that the Romney plan formed the basis for the Democrats' effort justifies a rejection of Romney by Republicans because he would be weak on the issue in an election matchup with Obama on November 6 - the weakest candidate we could possibly put forward.

This kind of message is resonating with some conservatives like John Barr, 50, of Circleville. Barr said he would reluctantly vote for Romney if he were the nominee and instead wants Santorum.

I want to repeal Obamacare. I believe in a free market solution and I don't believe in any kind of government solutions to our healthcare (system). If that law continues everyone loses, he said.

Romney has largely kept his focus on Obama's handling of the U.S. economy and foreign policy in the run-up to Super Tuesday and has been joined by his wife, Ann, who came armed with zingers of her own.

There's only one answer, and it's right here. If Mitt wins, America wins. If Mitt loses, America loses, she said in Zanesville.

Romney himself questioned the ability of Santorum, a veteran of Washington politicking, to deal with the economy.

We need to have a president who understands the economy if we're going to fix the economy. And my understanding of the economy and jobs did not come by reading about it or debating it in a subcommittee meeting, Romney said. My experience in the economy actually came by living in the economy.

Obama hoped to grab some of the Super Tuesday spotlight for himself by scheduling a rare White House news conference.

(Additional reporting by Eric Johnson; Editing by Eric Walsh)