With Tuesday's Republican primary in Michigan underway, polls in the state show a momentum shift toward former Pennsylvania senator Rick Santorum, according to interviews conducted Monday in the state by Public Policy Polling, a pollster affiliated with the Democratic Party.

The firm's results showed Santorum leading Mitt Romney by 5 percentage points, 39 percent to 34 percent, with a margin of error that represents a net 7-point reversal from interviews conducted Sunday that gave the former Massachusetts governor an advantage of 2 percentage points -- 39 percent to 37 percent.

Arizona also holds its Republican primary Tuesday. Both it and the Michigan contest are a warmup to the delegate-rich voting in next week's (March 6) Super Tuesday contests in  10 states.

Speaking Tuesday morning in Livonia, Mich., Romney argued that he has struggled in attempts to appeal to his party's right wing because he is unwilling to make incendiary comments, the Associated Press reported.

He also accused Santorum of trying to kidnap the presidential nominating process with automated calls urging Democrats to vote in Tuesday's primary.

On Monday, Santorum's campaign was actively seeking the support of Democrats in the Michigan primary, running a robocall with a script, obtained by the liberal blog Talking Points Memo, that read: Michigan Democrats can vote in the Republican primary on Tuesday. Why is it so important? Romney supported the bailout for his Wall Street billionaire buddies, but opposed the auto bailout. That was a slap in the face to every Michigan worker. And we're not going to let Romney get away with it.

The call closes with: This call is supported by hard-working Democratic men and women and paid for by Rick Santorum for President.

As the Wall Street Journal noted, Michigan's primary is more than a test of Romney's ability to win his native state. It is a test of his electability argument -- that Romney has the best chance of beating President Barack Obama.

The results will show whether Romney -- the longtime, if much-maligned, Republican front-runner who is now locked in a close race nationally with Santorum -- can win the blue-collar voters who are a pivotal bloc in the swing states of the Midwest.

Michigan also will test whether Romney, whose father served as the state's governor in the 1960s, can appeal to Republicans' core social conservatives and Tea Party movement supporters who have resisted him in previous contests.