By Eric Johnson

DETROIT, Feb 27 (Reuters) - Republican social conservative Rick Santorum could see an uptick in support during Tuesday's presidential primary in Michigan from an unlikely voting bloc: hardcore Democrats.

Democratic activists and strategists have launched a campaign to push fellow Democrats and independents to vote for Santorum to try to derail the more moderate frontrunner Mitt Romney, a Michigan native and the candidate President Barack Obama's campaign least wants to face in the Nov. 6 election.

"I think Santorum is completely radioactive and will bring an electoral disaster to the Republicans - he could deliver Obama a landslide," said Michigan Democratic strategist Joe DiSano, who has launched one of the efforts to help Santorum. "We need to focus on the one real challenger to Romney."

The Republican vote is an open primary, meaning Democrats and independents can vote in it too. Roughly 850,000 people voted in the Republican primary in Michigan in 2008. Democrats made up 7 percent of the Republican primary electorate in 2008 in the state, according to exit poll data. This time around they could help Santorum collect many of Michigan's 30 delegates and give him a big win.

Roughly 12,000 Democrats expressed interest in voting for Santorum when they were contacted during a "robo-call" phone campaign organized by Democratic activists, DiSano said. But it is not clear how many will actually turn out on Tuesday.

The Democrats want to underscore how weak Romney is in Michigan, the state where he grew up and where his father was a popular governor. Still, Romney is running neck and neck with Santorum in polls.

Santorum, a former senator from Pennsylvania, is a staunch opponent of abortion rights and gay marriage. He often makes inflammatory statements on social issues and is seen as much less likely to beat Obama in the general election.

"If Santorum wins the Michigan primary, Romney has some serious problems," said William Rose, a professor of political science at Albion College in Southwest Michigan.

"Romney will win Arizona easily, but the Republican Party leadership will start to panic even more than they already are. The stakes are very high."

Democratic Party officials in Michigan said they have encouraged crossover voting - but not specifically for Santorum.

"There has been some heavy social media activity here in Michigan - Internet, email, Twitter, Facebook - but we are not part of it," said Michigan Democratic Party chairman Mark Brewer. "Clearly these activists are making a judgment that it is better to face Santorum. That is their judgement, not mine."

A Romney campaign spokesman, Ryan Williams, accused Santorum of inviting Democrats to vote for him, a strategy he called "outrageous."

He said Santorum "is now willing to wear the other team's jersey if he thinks it will get him more votes." (Editing by Alistair Bell)