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.
Santorum needs an Ohio win to prove he remains a threat to Romney after losing Michigan and Arizona to him last week.
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.
Senior Romney aide Eric Fehrnstrom was cautious about Romney's prospects in Ohio, saying the campaign was more concerned about getting enough delegates to eventually seal the nomination.
"I don't think any state is a must-win. I think the only must-do on a candidate's checklist is getting 1,144 delegates," he told reporters on Romney's plane.
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."
Republican Senator Tom Coburn of Oklahoma, a fiscal conservative, also endorsed Romney. Oklahoma votes on Tuesday as well and Santorum has been in the lead there.
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.