NEWTOWN, Ohio — Republican presidential candidate Mitt Romney trained fire at Rick Santorum on Monday as he opened a crucial week of campaigning looking to claw back ground lost to his surprise rival.

Romney, the former Massachusetts governor who was long the front-runner, finds himself struggling ahead of key contests in Michigan and Arizona on Feb. 28 and the March 6 “Super Tuesday” when 10 states vote.

Santorum, a former senator from Pennsylvania, has emerged in recent weeks as the main conservative alternative to the more moderate Romney in the battle for the right to face Democratic President Barack Obama in the Nov. 6 election.

Santorum is posing a serious threat to Romney with a blue collar-themed message that is finding some appeal in economically hard-hit Midwestern states.

Romney, speaking at a bioscience company in Ohio, a key Super Tuesday state, attempted to raise doubts about Santorum's tenure in Washington and allay concerns about his own candidacy among conservatives.

Romney said he is a fiscal conservative with a career in business, while Santorum voted to raise the U.S. debt ceiling and supported government spending projects deemed wasteful by critics.

“One of the people I'm running against, Senator Santorum, goes to Washington, calls himself a budget hawk, and then after he's been there a while says he's no longer a budget hawk. Well, I am,” Romney said.

“He voted five times to raise the debt ceiling without getting compensating cuts in the spending. When Republicans go to Washington and spend like Democrats, you're going to have a lot of spending, and that's what we've seen over the last several years,” he said.

An average of recent polls gives Santorum a lead over Romney in Ohio of 33 percent to 26 percent. In Michigan, Santorum had built a big lead but a Public Policy Polling survey said Santorum now leads Romney in the state by a small margin, 37 percent to 33 percent.

But experts say Romney has time to turn it around. He will get a big opportunity on Wednesday in Mesa, Arizona, when the four remaining Republican candidates - Romney, Santorum, Newt Gingrich and Ron Paul - gather for their 21st and possibly last debate.

Many pundits see Michigan as a linchpin for Romney because it is the state where he was born and where his father was governor. A loss there would be a heavy blow coming before Super Tuesday. Romney is to visit Michigan on Tuesday, holding a town hall in a Detroit suburb.