Santorum responded with a commercial in Michigan designed to blunt the attacks and tarnish Romney. It shows a Romney lookalike wielding a machine gun that sprays mud in Santorum's direction.
When the weapon jams, the gunman tries to fix it, and ends up splattering himself instead.
The Red White and Blue Fund, which supports Santorum, is advertising in Michigan, but the former senator and his allied group are being outspent roughly 3-1 by Romney and Restore Our Future.
"We know that we're going to win some and were going to lose some, but we have the resources and the organization to go the distance even if that means a primary calendar that extends into the spring," said Gail Gitcho, communications director for Romney.
With Gingrich unable to come close to duplicating his Jan. 21 victory in South Carolina, his goals are as diminished as his campaign bank account.
"We want to aim at Washington state with 40 delegates. We want to do as best as we can in Michigan, but understanding there's a larger strategic gain for us if a consensus builds around the idea that Mitt Romney is unacceptable as the nominee," campaign spokesman R.C. Hammond said.
He said Ohio, Georgia, Oklahoma and Tennessee are priorities.
A state-by-state list, with the number of delegates at stake in parentheses:
Arizona primary (29): The winner gets all the delegates, and private polling shows Romney well ahead. Candidates gather in Mesa on Feb. 22 for their first debate in three weeks.
Michigan primary (30): The relative lack of suspense about Arizona heightens the political significance of Michigan, the first of the big industrial states to vote in the Republican race. Romney, who grew up in the state, won it four years ago. Santorum's support in the polls is rising statewide as well as nationally, and he hopes for an upset that can strengthen his chances on Super Tuesday.
Washington caucuses (40 delegates): Santorum hoping for a victory. Three delegates go to the winner of each of the state's 10 congressional districts, an invitation for strong competition.
March 6 (Super Tuesday, seven primaries, three caucuses, 419 delegates total)
Alaska caucuses (24): Delegates are awarded in proportion to the statewide vote. Paul may fly there in search of an elusive victory.
Georgia primary (76): Gingrich's home state when he was in Congress, and anything other than a victory would resurrect talk of a campaign exit.
Idaho caucuses (32): A large Mormon population makes this a natural fit for Romney. Santorum campaigned there last Wednesday, Paul on Friday.
Massachusetts primary (38): Romney could win all of the delegates in his home state.
North Dakota caucuses (28): Santorum made three stops in the sparsely settled state in a single day recently, and hopes to add it to his list of earlier caucus victories in Iowa, Minnesota and Colorado. Paul is also hoping for success.
Oklahoma primary (40): Private polling makes this a three-way toss-up among Romney, Santorum and Gingrich, who's targeting it as part of a Southern-based revival strategy.
Ohio primary (63): A big battleground state, although the results of the Michigan primary on Feb. 28 are likely to reset the race instantly. As elsewhere, Restore Our Future, the pro-Romney Super-Pac, got the jump in television advertising.
Tennessee primary (55): One of the states Gingrich hopes will launch a comeback, and polling currently shows a competitive three-way race in a state that allocates delegates in proportion to the popular vote.
Vermont primary (17): The second New England state on the ballot, and the one with the fewest delegates of all the Super Tuesday states. Romney is favored although the delegates could be divided.
Virginia (46) : Romney figures to get all the delegates for little effort, with neither Santorum nor Gingrich on the ballot.
Associated Press writer Stephen Ohlemacher contributed to this report.
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