By Steve Holland and Jeremy Pelofsky

WASHINGTON (Reuters) - Mitt Romney closed in on Rick Santorum in Ohio and picked up a crucial endorsement in Virginia on Sunday as he grows in strength ahead of "Super Tuesday," the biggest day yet in the race for the 2012 Republican presidential nomination.

A Reuters/Ipsos poll found Romney, a former Massachusetts governor, is tied with Santorum at 32 percent support from likely voters in the Ohio Republican primary, the most important of the 10 state nominating contests on Tuesday.

Santorum, a former Pennsylvania senator who is a social conservative with a strong blue-collar message, had been leading in polls in the economically hard-hit Midwestern state by double digits in recent weeks and hopes to score an important victory there.

But Romney's five straight victories in Maine, Michigan, Arizona, Wyoming and Washington state have given him a burst of momentum despite some serious reservations about him from conservatives who have flirted with a variety of alternatives.

Adding to his momentum in the race to become the challenger to Democratic President Barack Obama in the November 6 election, Romney picked up the endorsement of Eric Cantor, the conservative No. 2 Republican in the House of Representatives.

Cantor is from the Super Tuesday state of Virginia, where only Romney and libertarian Representative Ron Paul are on the ballot because Santorum and former House Speaker Newt Gingrich failed to meet all the requirements for qualifying.

"He is the guy I believe that will be our nominee and we will have a clear choice as a country as far as the vision forward in growing this economy with Mitt's plan versus that of the president's record," Cantor said on NBC's "Meet the Press."

An NBC News/Marist poll on Sunday gave Santorum support of 34 percent of likely Ohio primary voters while Romney got 32 percent.

Santorum has emphasized his social conservative views on contraception and abortion that arguably have distracted attention from his economic message.

SPLITTING THE CONSERVATIVE VOTE

Speaking on "Fox News Sunday," Santorum suggested the presence of another conservative in the race, Gingrich, was holding him back because it is splitting the conservative vote.

He stopped short of calling on Gingrich to pull out of the Republican nomination race.

Asked about Ohio, Santorum said he would do "very, very well" but did not predict victory.

"It's a tough state for us only because of ... the money disadvantage. But we've got a great grassroots campaign. We're hanging in there," Santorum said.

"It's always harder when you've got two conservative candidates out there running in the race. ... We have the anti-Romney vote, if you will. Both Gingrich and I are out there slugging away. We just need to show that we are the best candidate to go head to head," he said.

Super Tuesday offers something for Romney and all his remaining competitors as they search for a path forward. A rich trove of 419 nominating delegates is available; a total of 1,144 delegates is needed for the nomination.

Gingrich is placing his hopes on a victory in his home state of Georgia to get him back in the 2012 race. He leads there by double digits.

Gingrich had a brief period as the front-runner but was staggered by a loss to Romney in Florida on January 31 and has been searching for a comeback since then.

"I thought it was vital to the campaign, and we focused on it and as a result despite a lot of money spent against me, we're doing very well and I think we're going to win decisively," Gingrich told CNN's "State of the Union."

(Additional reporting by Will Dunham and Jeff Mason; editing by Alistair Bell and Mohammad Zargham)