Mitt Romney is "embarking on a make-or-break week in his bid for the Republican presidential nomination," says Matthew Jaffe at ABC News. Until very recently, he was considered a lock to win the Feb. 28 primaries in Michigan and Arizona. Now, the polls have him neck-and-neck or losing to social conservative Rick Santorum. If Romney can eke out victories in these two tricky states, "the nomination will again be his to lose," and he'll head toward March 6 Super Tuesday with whatever pun on "momentum" the media coins. But what happens if he loses? Here, four scenarios:

1. The Republican elite would dump Romney
If Romney loses even just Michigan, but especially both states, "heartburn is way too mild a term to describe what would happen inside Republican circles," say Jill Lawrence and Alex Roarty at National Journal. If a huge fundraising advantage, superior organization, and the backing of the GOP establishment aren't enough to help Romney win his birth state and Mormon-heavy Arizona, Republicans will flee to Santorum or...

2. Conservatives would hunt for a "white knight"
Very few people realistically think "Santorum is a likely nominee even if he wins Michigan," says Jack Lessenberry at Michigan Radio. A more likely scenario, veteran campaign watchers say, is that "national GOP strategists like Karl Rove would intervene to get some new candidates into the race — Chris Christie, for example, or Indiana's Mitch Daniels." Or Jeb Bush, an unidentified GOP senator tells ABC News. Clearly, "if Romney cannot win Michigan, we need a new candidate" — lest we "get killed" in November.

3. Romney would have to fight all the way to Tampa
The only way in for a white knight like Daniels or Christie is a risky, chaotic brokered Republican convention in Tampa, says John Dickerson at Slate, so that scenario is, I'd argue, highly unlikely. Here's how I see it unfolding: All four actual candidates slog through to the August convention and reach a deal beforehand on how the frontrunner will get enough delegates to win, and what the also-rans will get in return.

4. Mitt would have to self-finance
Romney is already burning through money faster than he's raising it, spending heavily to get one surging competitor after another out of the race. If his massive spending can't bury Santorum on Tuesday, Romney faces a long fight against two billionaire-backed rivals, at a time when his own wealthy donors are maxing out their contributions. He may have to "dig into his own pockets to bankroll his campaign," says ABC News' Jaffe. Count on it, says Philip Klein at The Washington Examiner. Romney has the bank account to do it, but remember, self-financing would "undercut one of his electability arguments — that he's a great fundraiser" — and underscore "his difficulty galvanizing grassroots support."