The Santorum campaign confirmed it began automated phone calls to Democrats Monday night. The calls said: "Romney supported the bailouts for his Wall Street billionaire buddies but opposed the auto bailouts. That was a slap in the face to every Michigan worker and we're not going to let Romney get away with it."
The appeal to Democrats -- who are allowed to declare themselves temporary Republicans on the spot Tuesday and vote in Michigan's open primary -- came as two polls showed the former Pennsylvania senator and former Massachusetts governor in a statistical dead heat.
A Mitchell/Rosetta Stone poll of likely Republican primary voters, taken Sunday, put Santorum 2 percentage points ahead of Romney, 37 percent to 35 percent. The phone poll of 858 likely primary voters has a 3.34 percentage point margin of error.
Former U.S. House Speaker Newt Gingrich of Georgia and U.S. Rep. Ron Paul of Texas, are locked in a battle for third place, with 9 percent and 8 percent, respectively, the poll found. Eleven percent of respondents remain undecided.
A separate Rasmussen Reports phone survey of likely Republican primary voters, also taken Sunday, finds Romney with 38 percent support to Santorum's 36 percent.
The phone survey of 750 likely primary voters has a 4 percent margin of error.
Paul and Gingrich remain behind with 11 percent and 10 percent, respectively, the poll found. One percent like another candidate and 5 percent remain undecided.
A Romney spokesman told The New York Times he considered the Santorum robocall effort to woo Democrats "unreal" and "outrageous" and said Santorum was "now willing to wear the other team's jersey if he thinks it will get him more votes."
"We believe that Republicans will decide who wins Michigan -- and we are confident that will be Mitt Romney," the spokesman told the newspaper.
There were reports the United Auto Workers members were actively but informally talking about voting against Romney. UAW President Bob King said in a statement the union had "no organized effort" to encourage members to vote Tuesday.
Thirty delegates to the Republican National Convention in Tampa, Fla., in August are at stake in Michigan.
In the Arizona primary Tuesday, which Romney is widely favored to win, 29 delegates are at stake.
Republican Arizona Gov. Jan Brewer endorsed Romney Sunday, telling NBC's "Meet the Press" he was "the man that can carry the day."
Of the state's 1.1 million registered Republicans, more than 250,000 submitted early ballots, and most did so before Santorum caught up with Romney in national polls, Fox News Channel reported.
The Washington state caucuses and straw poll will be conducted Saturday, with 40 delegates in play, followed by Super Tuesday March 6, when 10 primaries and caucuses award a total 437 delegates.
Super Tuesday involves contests in Alaska, Georgia, Idaho, Massachusetts, North Dakota, Ohio, Oklahoma, Tennessee, Vermont and Virginia.
A total of 1,144 delegates are needed to win the party nomination at the convention.
Before Tuesday, Romney had 123 delegates, Santorum had 72, Gingrich 32 and Paul 19.
In Monday campaigning, Romney told a Rockford, Mich., audience near Grand Rapids Santorum was "a nice guy, but he's never had a job in the private sector."
By contrast, Romney said he had "spent 25 years in the business. I understand what happens to corporate profit -- where it goes if the government takes it. This is what I've done throughout my life."
Romney grew up in Michigan, and his father, George W. Romney, was chairman and president of American Motors Corp. from 1954 to 1962 and the state's governor from 1963 to 1969.
Santorum told a crowd, "You know we have one of the candidates who claims a lineage here in the state of Michigan, although I do believe he was governor of Massachusetts, not Michigan."
He said being derided by Romney, who Santorum associated with Democratic policies, was "a joke" and he told Michigan voters they had an "opportunity to stop that joke" Tuesday.