Mitt Romney enters today...in excellent shape. To be sure, he's already lost out on the goal of sweeping to an easy nomination in January and he's been forced to fight through Super Tuesday...but that happened back in South Carolina; yesterday doesn't change that. Romney has a comfortable delegate lead (although it's far too early for that alone to be decisive). He's been in the lead more often than not in national polls. He has a huge lead in money and endorsements. And recall (and yes, there are asterisks and loopholes on some of these):
Bob Dole in 1996 lost four of the first five states.
George W. Bush in 2000 lost three of the first seven.
John McCain in 2008 lost four of the first six, and five of the first eight.
The calendar continues to help him. He's presumably going to win or finish second to Ron Paul in Maine; either way, he'll pad his overall delegate lead (although those delegates won't be chosen yet). Then, winner-take-all Arizona and winner-take-many Michigan on February 28, both states that should play very well for the Mittster.
What that means is what we've known for a while, but with slightly more uncertainty than before. That is, Mitt Romney will (almost certainly) head to Super Tuesday with a commanding grip on the nomination. He'll have many advantages on that day. If he sweeps the day or close, it will be plain to everyone that the nomination is decided. If he has an OK day, he'll still have a commanding lead. And if he has a bad day...then he's still a clear leader for the nomination, but he enters the 1984 Mondale territory that Nate Silver sees likely. Again: all of that has been true since South Carolina.
The uncertainty? Maybe things go wrong on February 28. Maybe he has an awful day on Super Tuesday (winning only Massachusetts and Virginia). But none of that seems very likely. The truth is that Romney didn't campaign much in yesterday's states, and that's not going to happen in the next round. Jonathan Chait got this right: Santorum bragged last night about the results proving what happens when he's not outspent 5 to 1 or more by Romney, but the truth is he will be outspent by that much in every key state and most other states going forward.
Yes, I still think that Santorum is a lot more plausible as a nominee than Gingrich, but even after yesterday, he's farther behind now than he was on January 1. As I said last night, he's going to have to show a lot of things that he just hasn't shown yet to move up to the point where he'd be a serious threat to win the nomination. Maybe that will happen; he's certainly surprised everyone (including me) before, so it's hard to write off even the possibility.
But the real thing to remember is that if the question we're interested in is just who will be the Republican nominee for president, then it isn't about winning every state, or winning every region, or winning with every constituency. It's about winning the nomination. And for that, it still sure looks like it's going to take some unexpected external shock for anyone but Romney to win it.