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

Romney, a former Massachusetts governor and venture capitalist, already has a big lead in the number of delegates accumulated in primaries and caucuses held so far, and he is trying to project confidence.

"I hope that I get the support of people here in Ohio tomorrow, and in other states across the country. I believe if I do, I'll get the nomination," he said.

Santorum, a former Pennsylvania senator, is going all out in Ohio in an effort to regain the momentum that slipped away in his loss to Romney in Michigan last week. He had held a sizable lead in Ohio but Romney has closed that gap.