All Super Tuesday eyes are on Ohio
Mitt Romney hopes to use Super Tuesday's massive batch of primary contests in 10 states to reassert himself as the unchallenged front-runner in the race to challenge resident Barack Obama.
On the eve of the biggest day of voting so far Mr Romney had picked up key endorsements from Eric Cantor, the second-ranking Republican in the House of Representatives, and Tom Coburn, a fellow fiscal conservative.
At stake in Tuesday's voting are 419 out of 1,144 delegates to the party's national nominating convention. The outcome could again reshape the battle which has seen several candidates, Rick Santorum being the latest, make a serious run at Mr Romney.
The former Massachusetts governor is gaining momentum, following his fourth straight victory in Saturday's low-turnout Washington state caucuses.
Many of the most conservative Republicans distrust Mr Romney's ideological purity because of his moderate positions in the past on such key issues as abortion, gay marriage and health care reform.
With Mr Santorum now leading a charge based on an intensifying debate over conservative social values, those issues are threatening to overshadow an emphasis on the economic concerns of Americans that could be the key to winning the November election.
Mr Obama is most vulnerable on the economy, which has struggled throughout the first three years of his presidency, following the near collapse of the US financial system in the last months of the George W. Bush presidency.
Super Tuesday's defining contest may be in the big Midwestern industrial state of Ohio, where Mr Santorum and Mr Romney have devoted tremendous time and resources in recent weeks. Mr Santorum's performance there could well define his fate - and Mr Romney's - in the roller coaster race going forward.
Preparing for the worst, Mr Romney's campaign began preparing for a possible loss in Ohio, where polls show him locked in a dead heat with Mr Santorum.
A win by the overmatched Mr Santorum would send a broad signal that Mr Romney, long presumed the front-runner, is far weaker than imagined.