Although Romney and Santorum each are far ahead of Newt Gingrich, with a pair of elections as close as these may be, the question of what Gingrich's continued presence in the race will play a role in the outcome of each state looms large. According to PPP, Santorum's 4 point Michigan lead would expand to 9 points, and Romney's 3 point Arizona lead would become a 2 point Santorum lead in Arizona. Thus, Gingrich's continued presence in the race is costing Santorum about 5 points in the polls.
But there's a difference between the polls and election day: Newt Gingrich supporters who also dislike Mitt Romney but could accept Rick Santorum will probably realize that a vote for Newt is a wasted vote, since he won't get enough support in either state to win any delegates. Similarly, Gingrich supporters who dislike Santorum but could accept Romney will also be likely to switch. There's clearly more of the former than than the latter, but the reality of Gingrich's abysmal polling will probably catch up with him, and Santorum will benefit. In the end, it could still shrink his net margin by two or three points, but I doubt it will be the full five that current polls are showing.
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Still, two or three points could be the difference between winning and losing, and if the race between Santorum and Romney ends up being that close in either state, Gingrich will have given a big boost to Romney—at least in terms of the final results in those states. But while Newt Gingrich's presence is helpful to Romney in Michigan and Arizona, that doesn't mean that Gingrich's presence is guaranteed to help Romney overall.
Certainly, in most conventional scenarios, Gingrich would help Romney going forward, but don't forget everybody was saying the same thing about Rick Santorum's presence in the race just a few weeks ago—and now look where things stand. If Gingrich were to withdraw and endorse Santorum, it would certainly give Santorum a short-term boost—perhaps a big enough one to win the nomination. But if Gingrich were out of the race, Romney wouldn't have to worry about the possibility of a Newt comeback if he destroyed Santorum. So while Romney would rather split his opposition than lose to a single candidate, at some point he's going to need the field to narrow to be in a position to win.
I'm hedging a bit here, but that's because I don't think it's possible to predict with certainty what will happen in this primary. Obviously, if on Super Tuesday, Newt Gingrich's presence on the ballot helps Mitt Romney win some states he would have otherwise lost to Rick Santorum, Romney would be thrilled. But it also seems plausible that Gingrich could win a state like Georgia without costing Santorum any states, and if that were to happen, Romney would have to confront the reality of having been beaten by not one, but two different candidates on the day that pundits expected him to wrap up the nomination.
Whatever the political impact of Gingrich's presence (or lack thereof) in the race, according to PPP's poll of Michigan, Romney backers are more likely to want Gingrich to quit the race than any supporters of either Rick Santorum or Ron Paul. Sixty-five percent of his supporters want Gingrich out and just 24 percent want him to continue, a 41 point margin. In contrast, 42 percent of Santorum voters want Newt out compared with 40 percent who want him to continue. The bottom line, whatever the political consequences: 72 percent of Republicans who don't like Newt want him out and 69 percent of the ones who like him think he should stay in.