WASHINGTON (AP) - On the eve of their Super Tuesday showdown, Mitt Romney and Rick Santorum strained for an edge in Ohio on Monday and braced for the 10 primaries and caucuses likely to redefine the race for the Republican presidential nomination.

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Newt Gingrich, though winless for more than a month, campaigned in Tennessee and issued a stream of signals that he intended to stay in the race.

In a race marked by unpredictability, Romney's superior organization and the support of an especially deep-pocketed super PAC allowed him to compete all across the Super Tuesday landscape and potentially pick up more than half of the 419 delegates at stake.

Santorum cast the race in biblical terms, his David vs. Romney's Goliath. Even that "is probably a little bit of an understatement," he added.

Primaries in Ohio, Georgia, Massachusetts, Vermont, Virginia, Oklahoma and Tennessee plus caucuses in Idaho, North Dakota and Alaska make Tuesday the busiest day of the primary season.

Unlike previous Republican campaigns, when a primary winner would typically win all of a state's delegates, allocations this year generally reflect the split in the popular vote.

As a result, several candidates may be able to claim success once the Super Tuesday results are known. Romney kept his focus on the economy in a final sprint across Ohio, the state that has drawn the most attention and television advertising.

Pre-primary polls show him with momentum in a close race with Santorum.

"Other people in this race have debated about the economy, they've read about the economy, they've talked about it in subcommittee hearings," Romney said dismissively of his opponents. "But I've actually been in it. I've worked in business, and I understand what it takes to get a business successful and to thrive."

Santorum, who narrowly lost Michigan to Romney last week, said that no matter how much his rival spends, "conservatives will not trust him, will not rally around him this primary season. ... We will be the nominee."

He said he looked forward to the day when Gingrich drops out and clears the way for him to challenge Romney one-on-one. "And when we do that, we'll win," Santorum said.