Let's get the most important thing out of the way first: Romney is still going to win the nomination. Super Tuesday might not have created the sort of unambiguously positive buzz he was looking for, but he escaped the day with something far more important: a commanding delegate lead. Momentum and buzz are ephemeral; delegates are forever.
But is there a scenario in which Romney doesn't win? Or, barring that, which scenario brings him the closest to losing?
It's an interesting question because the two possibilities are diametrically opposed. And it all depends on Newt Gingrich.
Scenario A: Newt Gingrich, finally exhausted of running a pointless candidacy, drops out.
Why it might hurt Romney: It lets the voters who oppose Romney finally coalesce around one candidate. For the first time in the entire campaign, Romney would have a single challenger. However small a plurality Romney commands, he still looms over his squabbling opponents. Santorum, as the last Real Republican standing, might attain a prominence that finally lets him compete with Romney on a relatively even field.
Why it might not: At the end of the day, Gingrich dropping out still brings Romney one step closer to surviving the primary. Romney would probably extend his delegate lead (see below), and he might get some good press out of the deal, too. Generally speaking, when the only thing standing between you and the presidential nomination is Rick Santorum, you're in good shape.
Scenario B: Gingrich, driven by a manic kamikaze zeal that will not fade until he has been rejected by every last Republican in North America, remains indefatigable, and pours millions of dollars more into the burning wreckage of his campaign.
Why it might hurt Romney: Because of the voters Gingrich is continuing to capture, a certain percentage would probably switch to Romney after Gingrich dropped out. Gingrich is therefore likely skimming some small number of delegates away from Romney, and keeping Romney's final total lower than it would otherwise be. In the most extreme case--which is, frankly, not going to happen--Romney is somehow prevented from winning an outright delegate majority. More likely, it prevents Romney from winning his majority until the far end of the race, shortening the general election and subjecting him to intra-party attacks for months to come.
Why it might not: However long he prolongs the election, Gingrich has performed a valuable service for Romney: he divided the Tea Party and kept the conservative wing of the GOP from winning a number of races it might have otherwise. Absent Gingrich, I find it doubtful Romney would have eked out victory in Ohio, Iowa, and maybe Michigan. Whatever delegates Romney has lost to Gingrich, Newt has also helped keep anti-Romney sentiment from snowballing. And to whatever extent anti-Romney sentiment might still snowball, Mitt should be glad Newt's still in the race.
Which scenario is more likely to hurt Romney? And which one is more likely, period? I'm leaning slightly towards Scenario B for both questions. Newt Gingrich is a megalomaniac and I just can't imagine him throwing in the towel. But if history is any guide, Mitt should want him to. I remember 2008. In particular, I remember the way that, as the Democratic primary drug on, the media campaign began mattering less and less, and the delegate math began mattering more and more. The larger Romney's delegate lead, the more he can ignore the daily campaign drama and plan for November. Too bad, because Newt's not going to give up his delegates without a fight.