ABC News' Michael Falcone reports:

ROYAL OAK, Mich. - How badly does  Rick Santorum want to win Tuesday's Republican primary in Michigan?

So badly his campaign is making an appeal to Democrats on primary eve. Santorum, who is locked in a tight race with Mitt Romney here, has paid for robo-calls inviting Democrats to cross-over and vote for him.

The call began hitting Michigan households on Monday.

"On Tuesday join Democrats who are going to send a loud message to Massachusetts Mitt Romney by voting for Rick Santorum for president," an announcer says.

Santorum aides say it is an attempt to swing "Reagan Democrats" their way, however it is much more likely that any cross-over votes they may get will come from the liberal rather than moderate wing of the party.

Markos Moulitsas, founder of the progressive blog, the DailyKos, has been urging Democrats in Michigan to vote for Santorum in the state's open primary in the hopes of denying Romney a win in his home state.

"They're allowed to vote," Santorum said of Democrats while campaigning in Kalamazoo, Mich. Monday night. "And anybody who wants to vote, I encourage them to vote.  Why wouldn't we encourage people to vote for me?  We got a plan - we want to encourage folks to show that we can win and get a bipartisan vote."

Santorum adviser Hogan Gidley echoed his candidate's comments, saying in an interview with ABC News, the robo-calls were an attempt to broaden Santorum's base.

"Rick Santorum has been consistent that we're going to need a motivated an energized conservative base and Reagan Democrats if we're going to beat Barack Obama in the fall," Gidley said. "We just figured if we can get them now, then they're going to be there with us in the fall."

The Romney campaign sees the effort differently.

"It is outrageous that Rick Santorum is inviting Democrats into the Republican primary to vote against Mitt Romney," Romney spokesman Ryan Williams said in a statement. "Rick Santorum has moved beyond just 'taking one for the team,' he is 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."

Here's the script of the call, which was first reported by Talking Points Memo:

"Michigan Democrats can vote in the Republican primary on Tuesday. Why is it so important? 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. On Tuesday join Democrats who are going to send a loud message to Massachusetts Mitt Romney by voting for Rick Santorum for president. This call is supported by hardworking Democratic men and women and paid for by Rick Santorum for president."

ABC News' Shushannah Walshe contributed reporting.