Editor's note: Edward Morrissey is a senior editor and correspondent for the conservative commentary website HotAir.com.
(CNN) -- Voters in Arizona and Michigan are going to the polls Tuesday to make their choice for the Republican presidential nomination. But for Mitt Romney, it's Michigan that matters most.
A month ago, Romney had nearly insurmountable leads in both states. As voting opened Tuesday morning, he still had a large enough lead in Arizona that the leading newspaper in the state, The Arizona Republic, barely mentioned the primary on its front page. And the focus has fallen on Romney's native state of Michigan, where his father was once governor and which had been considered a bastion of support -- until Rick Santorum began connecting with blue-collar voters a few weeks ago.
Romney leads in assigned delegates by a large amount; officially, he has 73 bound delegates, while Santorum only has three. (Newt Gingrich, who won't be competitive in either of Tuesday's primaries, has 28.) Since Michigan allocates its 30 delegates proportionally while Arizona will assign all its 29 delegates to the winner, Romney will almost certainly extend his lead significantly no matter what the result in Michigan might be. Furthermore, with his fundraising and organization in place, he has a big advantage rolling into Super Tuesday seven days from now.
So why are the stakes so high for him in Michigan? Romney needs to win the state to maintain his argument for electability.
Going into Michigan, Romney had a home-field advantage, a large edge in fundraising and most definitely an overwhelming lead in advertising. Santorum helped Romney out six days ago in the CNN debate with a mediocre, defensive performance resulting from constant attacks by Romney during the event.
If Romney is truly both inevitable and the most electable candidate in the field, those advantages should have been decisive. Instead, surveys from Public Policy Polling show a rebound for Santorum in the last 48 hours, and the numbers indicate a narrow Santorum victory if that momentum continues.
Even Romney seems to recognize that a Michigan loss complicates his sales pitch. Earlier Tuesday, Romney allowed that "the candidate sometimes makes mistakes" and that "this isn't going to be over in a day or two." That sounds like a candidate attempting to manage expectations downward on a primary day -- although it should be noted that Santorum's team was singing a similar tune over the weekend.
Calling Michigan a "make-or-break state" for Romney might be an overstatement, but not by much. He obviously has the resources to go the distance in the nomination fight, a status that his competitors may not share at the moment.
Romney will win states on Super Tuesday next week, too. However, a loss in his native state after five years of campaigning and planning will damage Romney's argument for competence and electability, and breathe more life into the Santorum campaign after a week in which Romney should have delivered a knockout blow.
In this case, the impact goes beyond the delegate allocation.
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The opinions expressed in this commentary are solely those of Edward Morrissey.