The top two U.S. Republican presidential hopefuls are battling for Michigan Tuesday, in a key primary that once again may reset the course of an unpredictable nomination race.

Mitt Romney once held a commanding lead in the state where he was born and where his father was a popular governor, but is now fighting to fend off an embarrassing loss against his main challenger, Rick Santorum.

Santorum, a former U.S. senator, has surged in the polls coming off a three-state victory earlier this month in Colorado, Minnesota and Missouri.

Opinion surveys show Romney with a clear lead in the southwestern state of Arizona, which also is voting Tuesday to pick a Republican candidate to face President Barack Obama, a Democrat, in the November election.

Romney and Santorum could end up splitting Michigan's delegates because the state awards them based on the proportion of the vote. Arizona is a winner-take-all contest.

Romney, a former Massachusetts governor, is having difficulty winning the support of social conservatives, who are rallying behind Santorum.

Texas Representative Ron Paul and former House speaker Newt Gingrich are far behind in the polls. Gingrich turned his campaign efforts Tuesday to Georgia, a state he represented in Congress for two decades. Paul moved his campaign on to Virginia.

Romney and Santorum made last-minute appeals in Michigan for support, with campaign events across the state. Santorum is hoping a victory will cement his status as the conservative alternative to Romney, while Romney is seeking to boost campaign momentum and reclaim front-runner status.

The primaries come a week before the much-anticipated “Super Tuesday,” when 10 states hold presidential nominating contests.