Mitt Romney took an aggressive tone against GOP rival Rick Santorum at a Presidents Day speech in Ohio, as the Republican candidates geared up for a week of intense campaigning.

After touring a bioscience firm in Cincinnati, Romney underlined his business experience in a speech to employees.

Romney told the small audience he had run "four enterprises" and could better attack Washington's ballooning federal debt than Santorum.

"During his time in the senate, only two terms, the size of the federal government grew 80 percent," Romney said. "When Republicans go to Washington and spend like Democrats, you're going to have a lot of spending."

Romney said that unlike Santorum, he was a "budget hawk."

State and national polls show Santorum continues to pose a real threat to Romney, including in states such as Ohio, which votes March 6.

Romney will head next to Michigan, a state he has long been expected to win but where Santorum has proven to be a strong opponent.

The four GOP candidates will then meet on-stage at a CNN debate in Arizona on Wednesday.