Marc Lewis holds daughter Alexa, nine-months-old, as he votes at the Heritage Universalist Unitarian Church today in Cincinnati. Ohio is the biggest prize in political significance on Super Tuesday, the GOP presidential primary showdown across 10 states for more than 400 delegates. Mitt Romney and Rick Santorum have devoted much of their campaign attention to Ohio in recent days. (AP Photo/Al Behrman)

DAN SEWELL,Associated Press

CINCINNATI (AP) - Rick Santorum and Mitt Romney were locked in a tight race in Ohio, as expected in the hotly contested and politically important Super Tuesday state.

The former Pennsylvania senator led by only 1,824 votes, or 38 percent, with 11 percent of Ohio's precincts reporting unofficial returns. Romney, the former Massachusetts governor, had 37.15 percent. Santorum hoped to rejuvenate his bid to overtake Romney for the Republican presidential nomination by claiming his biggest primary prize yet, while Romney hoped to add another key November swing state to bolster his front-runner status.

Former House speaker Newt Gingrich was a distant third, at 15 percent, in the state that lies between Romney's native Michigan and Santorum's home state. Texas Rep. Ron Paul, who largely bypassed Ohio, was running at 8 percent.

Polls have tracked voter volatility among Ohio Republicans for months. Pizza magnate Herman Cain - who dropped out of the race in December - Gingrich and Santorum have all leapfrogged past Romney, only to fall back in the last six months. Late polls also indicated that significant numbers of likely Ohio primary voters said they might change their minds once they were casting their ballots.

Early results from an Ohio exit poll Tuesday found that many voters were undecided about a candidate until recently, with more than half saying they didn't decide until the last few days or weeks.

The state's geographic and economic diversity - cities, small towns, farmland and swaths of suburbs, along with Rust Belt manufacturing, agriculture, medical and high-tech businesses - made it a key test for the Republican contenders. No Republican nominee has reached the White House without carrying Ohio. President Barack Obama carried the state in 2008, after it delivered George W. Bush's clinching re-election margin in 2004.

Both candidates focused on Ohio in the last days before Tuesday's voting in 10 states. They crisscrossed the state and blitzed airwaves with ads. Santorum planned to watch returns in Steubenville, in eastern Ohio.

Gingrich made a weekend swing through Ohio, hoping to pick up some of the state's 63 delegates at stake. Even if Santorum wins the state, he faces leaving 18 delegates on the table because his campaign didn't get enough delegate candidates on all ballots.

Paul concentrated on states holding GOP caucuses Tuesday, although Ohio backers said he had pockets of support across the state. It's also relatively easy to cross over in Ohio's primaries, so Paul supporters wooed Democrats and independents, especially college-age ones.

Ohio has 66 Republican delegates total, including three party "super delegates" who aren't bound by the primary results.