Mitt Romney is calling Rick Santorum's calls to Democrats to vote in Tuesday's primary in Michigan "outrageous" and "disgusting" but Santorum says he's not doing anything besides getting people to vote in an open primary.

Romney complained about the automated calls Tuesday, as Michigan voters go to the polls in the Republican presidential primary. Arizona also votes Tuesday in a winner-take-all battle, but Michigan, with its proportional delegate award system, has added value since Romney was born in the state and his father served as governor there.

"I know why Obama doesn't want me to face him but I just think it's outrageous and a terrible dirty trick at the last hour, by the way, late in the afternoon on the day before the election, maybe hoping no one would notice, they start sending out calls to Democrats, union members telling them to go into the Republican primary and vote against Mitt Romney," he said.

"This is a new low for his campaign and that's saying something," Romney told Fox News

Santorum told Fox News on Monday night that he is trying to attract the Democratic voters he'll need in a general election campaign. He added that nothing he's said in his robo-call is any worse than the campaign Romney has run.

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"When he runs a robo-call of my voice from four years ago saying good things about him, that's not a low moment, and when I run a call basically saying, calling Democrats that are eligible to vote here, to vote for us, that's a low (moment)?" he said.

"And of course, you know, it's interesting that he criticizes me for attracting Democrats because one of the things that the Governor Romney's people say is, oh, he can't attract Democrats. Well, guess what? We will wait and see. I think we can."

The robocall going around Monday says Democrats should send "a loud message" to Romney by voting for Santorum. The message says it's supported by "hard-working Democratic men and women" and paid for by the Santorum campaign.

Indeed, Democrats have made no bones about their efforts to force a Romney loss in the GOP primary. The pro-President Obama super PAC, Moveon.org and the campaign itself have all run ads against Romney in the Wolverine State.

On Monday, liberal blogger Markos Moulitsas of the Daily Kos wrote that a "million-dollar anti-Romney effort (with much of that money coming from Democrats and progressives) has dragged Romney back down to where Santorum can catch him."

He added that "the big-money campaign" aims to drive down Romney's numbers among independents and the Michigan Democratic Party is also engaged in getting Democrats to cast votes for Santorum, "reminding them that casting such votes in no way prohibits them from voting in Democratic contests later in the year."

"The quicker Romney can transition to 'general election' mode, the quicker he can move to the center and start repairing the damage. And as long as Santorum drives the debate, he helps galvanize base Democratic constituencies. ... There is no downside to dragging this contest out a month or two longer," Kos wrote.

While the two candidates run neck-and-neck -- late polling showing virtually no difference between the two -- Santorum is also asking volunteers to come out Tuesday night to "help in ensuring the accuracy of ballots collected."

Whoever wins in Michigan, the 30 delegate count will only go a small way toward gaining the 1,144 required to win the party nomination at the Republican National Convention in Tampa, Fla.

So far, The Associated Press has tallied 123 delegates for Romney to 72 for Santorum. Newt Gingrich, who is focused on Washington state caucuses Saturday, with 40 delegates at stake and Super Tuesday states next week and not Tuesday's battle in Michigan, has 32 delegates and Ron Paul has 19. Next Tuesday's 10 primaries and caucuses will gin up another 419 delegates.

But Michigan's battle would follow a three-state sweep by Santorum earlier in the month, which while not awarding delegates yet, forced Romney into a bit of a defensive mode in Michigan. He hosted nearly a dozen public events as he and his allies have spent more than $2 million on local television advertising.

Romney predicted victory Monday night at the Royal Oak Music Theatre, where rocker-rapper Kid Rock performed.

"I'm going to win in Michigan and I'm going to win across the country," Romney said.

Santorum, seeing fluctuations in polling, was more reserved on Monday night.

"I think the fact that we are doing as well as we are is a pretty big deal in this state," he said.