Update, 10:14 p.m. ET: Mitt Romney and Rick Santorum slugged it out tonight in Arizona, but the importance was all about Michigan. Santorum, as the frontrunner in the polls, was on the defensive for most of the night. Romney, as the actual frontrunner, was playing it safe. The strategy paid off for Romney. During the course of the debate, his likelihood of carrying Michigan rose from 66.2 percent, 15 minutes before the debate, to 73.6 percent, at the close of the debate. And, in correlation, Romney's likelihood of gaining the GOP nomination is up over 3 points on the day to 75.7 percent.

Likelihood of Winning Michigan Primary During Arizona Debate1

Sources: Betfair and Intrade

Since we last convened for a good old Republican debate, Mitt Romney was convalescing from his loss in South Carolina and Newt Gingrich was quickly squandering his momentum. A month later, Romney's onetime death grip on the nomination has slackened, and he now faces a non-negligible threat to the nomination.

The chart shows the progress of the likelihood of gaining the