GRAND RAPIDS, Mich. (AP) — Mitt Romney is fighting to avoid an embarrassing home-state loss in a high-stakes Michigan primary that will not decide the Republican presidential contest but could scar the former Massachusetts governor just a week before Super Tuesday.

Rival Rick Santorum is calling upon an unusual coalition of tea party activists, religious conservatives and Democrats to help topple Romney and reclaim the momentum in the increasingly heated nomination battle.

Santorum, a former Pennsylvania senator, confirmed on the eve of the election that he had targeted Michigan Democrats with automated phone calls encouraging them to vote against Romney.

“Romney supported the bailouts for his Wall Street billionaire buddies but opposed the auto bailouts,” the phone call said. “That was a slap in the face to every Michigan worker and we’re not going to let Romney get away with it.”

Only declared Republicans may vote in today’s GOP primary, but party rules allow voters to change their affiliation temporarily on the spot. The potential involvement of Democrats adds a new twist to a contest already expected to have significant implications for Romney’s White House bid.

Neither Newt Gingrich nor Ron Paul is actively competing in Michigan’s contest, or in today’s Arizona primary, which Romney is expected to win handily, in part because of the state’s Mormon population.

Romney was born and raised in Michigan, where his father served as governor. But Santorum’s rise in polls following a three-state sweep earlier in the month has forced Romney to work hard in Michigan over the past week. He’s hosted nearly a dozen public events as he and his allies have spent more than $2 million on local television advertising.

Santorum will campaign today around Grand Rapids, a city set in a western Michigan region home to many social conservatives and tea party supporters. Romney is set to meet with voters 130 miles to the east in suburban Detroit, an area with a larger collection of moderate Republicans, a key segment of his support.

Romney’s overwhelming advantages in Michigan, however, may not pay off in a contest generally dominated by the Republican Party’s more conservative flank. He trailed Santorum by a significant margin in polls as recently as last week. In recent days, however, those polls have tightened, leaving today’s election essentially a tossup.