MikeLigalig.com

By John Whitesides

(Reuters) - The Republican presidential race faces a potential turning point on Tuesday in contests in Arizona and Michigan, where upstart Rick Santorum threatens to plunge an already unpredictable nominating battle into chaos.

Mitt Romney, the former front-runner and presumed nominee, and Santorum are in a close race in Michigan, the state where Romney was raised and his father was an auto executive and popular governor.

A Santorum win there would be a devastating blow to Romney, turning lingering doubts about his ability to win the allegiance of Republican primary voters into a deep panic in the party's senior ranks.

"If Santorum pulls off the upset in Michigan it turns the entire race on its head," said Republican Dan Schnur, an aide to Senator John McCain during his 2000 presidential bid. "There aren't going to be a lot of people who have much use for a front-runner who can't win his home state."