An aggressive Romney repeatedly put Santorum on the defensive in a CNN debate on Wednesday and attacked the former U.S. senator and staunch social conservative for supporting big-spending government programs.

Romney has also battled his way into a slight lead in a new poll in Michigan, which along with Arizona will hold a primary contest on February 28. Romney had trailed Santorum by as much as double digits a week ago in the Michigan race.

The pace of the Republican race quickens dramatically next month, with 22 state nominating contests in March, including 10 on "Super Tuesday" March 6.

"This debate will really give Romney some momentum heading into the Michigan primary," Republican strategist Ron Bonjean said. "Santorum clearly was having a hard time having to apologize and explain so many of his votes."

The debate was Santorum's first time in the political spotlight since charging into a national lead after winning three contests on February 7 in Missouri, Minnesota and Colorado.

But his sometimes awkward defense of votes he cast in the Senate, including his claim that politics was a "team sport" and that he sometimes had to vote for bills that did not like, could hurt his support among conservatives tired of Washington politics-as-usual.

"There is always an excuse," rival Ron Paul said of Santorum's various explanations. "That's what the problem is with Washington," said Paul, a Texas congressman who described Santorum's fiscal conservatism as "fake."

The primary in Michigan, where Romney was born and raised and his father was an auto executive and popular governor, has become a must-win for the former Massachusetts governor and former head of a private equity group.

A Romney loss there would be crippling blow that would set off alarm bells among Republicans about his ability to win the allegiance of conservatives, ensuring a long and potentially draining battle for the nomination.

But a win in Michigan and Arizona would put Romney back in command in a race that has featured a series of conservative rivals who have risen to challenge him only to fall back into the pack.

Santorum, a former U.S. senator from Pennsylvania, is the latest candidate to seriously contest the party's nomination to face Democratic President Barack Obama in a November 6 general election.

On Thursday, Romney will speak to a builders' group in Arizona before heading to Michigan for an evening rally of the conservative Tea Party movement. On Friday, he will address the Detroit Economic Club. Santorum is fund-raising in Texas on Thursday.