It may have been a Republican primary, but Democrat voters turned out in droves to vote for Rick Santorum in the Michigan primary, presumably attempting to topple Mitt Romney in his home state.

About 10 percent of the state's primary voters on Tuesday were Democrats, exit polls show. And they voted overwhelmingly for Santorum,with 53 percent picking the former senator versus the 17 percent who chose Romney.

Santorum actively courted Michigan's Democrats on eve of  the most pivotal election of the GOP primary race to date, paying for robo-calls asking liberal voters to "send a loud message to Massachusetts Mitt Romney by voting for Rick Santorum for president."

The Santorum campaign said the was intended to court "Reagan Democrats." But the Democratic voters that turned out are no "Reagan Democrats" who crossed party lines to vote their conscience. More than half - 57 percent - identified themselves as "liberal." Only 5 percent said they were conservative.

Santorum's was not the only Democrat-targeting robo-call to hit Michigan's phone lines today. A President Obama-supporting political consultant sent out a call at 5 p.m. reminding voters they only had three more hours to cast their ballots.

"Of course you want to vote for Rick Santorum," the voice of Tony Trupiano said in the call. "And make sure that vote is an embarrassment for Mitt Romney."

A blow to Romney it may have been. Without the Democrats' vote, Romney would likely have pulled an earlier and more substantial lead over Santorum.

But this party crossing mischief is not new to presidential primaries. In the last presidential election, when Barack Obama started to pull ahead in the Democratic primary race, conservative commentator Rush Limbaugh urged Republicans to vote for Hillary Clinton in any state with an open Democratic primary election. He called the scheme "Operation Chaos."

"[Obama] needs to be bloodied up politically," Limbaugh said on his radio show. "Since [John] McCain is not going to do it, since the Republican Party is not going to be on the field in this way, somebody's got to bloody up Obama.  The only person that can do it is Hillary, and she can't do it if she's not in the race."

The same story line unfolded in the 2000 presidential race.

In Michigan's GOP primary that year, 17 percent of the voters were Democrats and almost all of them - 82 percent - voted for John McCain.

ABC News' Gary Langer contributed to this report.