WASHINGTON -- Mitt Romney's campaign ratcheted up its attacks on Rick Santorum Tuesday morning, one week before voters go to the polls in Michigan's Republican presidential primary.

The Romney campaign sent out a release comparing Santorum, a onetime Pennsylvania senator, to former House Speaker Newt Gingrich, who was previously Romney's chief competitor in the race for the GOP nomination.

"IF YOU LIKED NEWT GINGRICH, WAIT 'TIL YOU GET TO KNOW RICK SANTORUM," the email screamed in the subject header.

Communications from the Romney campaign this week have tried to drive the message that Santorum's record and background are unknown, bear a closer look, and differ from the public image that Santorum cultivates for himself. The Gingrich comparison hinges on a few accusations flung by the Romney campaign: Santorum and Gingrich are both "career politicians" who "were pushed out of office by those who know them best," and both "moved to Washington D.C. and never left," even after they were out of public office.

"Mitt Romney offers something different. He is not a creature of Washington. He offers the kind of private sector and chief executive experience that career politicians can't match," said Romney spokesperson Andrea Saul in the release.

While the Romney campaign has been increasing the velocity of its negative attacks on Santorum for a few days, the comparison to Gingrich is the clearest sign that its focus now is on driving up Santorum's negatives, after spending the first week and a half following the Feb. 7 primaries and caucuses -- which Santorum swept -- trying to build up Romney's image with Michigan voters.

Most political observers, including some Romney supporters who have spoken with The Huffington Post privately and not for attribution, have said that if Romney loses the Michigan primary to Santorum next Tuesday, then Romney's candidacy will be badly, if not mortally, wounded. Many expect that Santorum would then become the clear frontrunner and would move toward clinching the nomination -- or another candidate might potentially jump into the race.

The move to full attack mode thus reflects an urgency at Romney's Boston headquarters to try to move the race in his direction decisively in the few days left before voters go to the polls in his home state.

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