John Dickerson says Mitt's powers of inevitability have been restored:

Romney emerges a stronger candidate. He certainly showed he was willing to do what it takes, for those voters who might worry about whether he was tough enough to go up against Obama. The terrain is now very favorable as Romney heads into contests in Nevada, Michigan, Colorado, and Minnesota. He won all four of those in 2008. The next contest where Gingrich might have a good chance is almost a month from now, on Super Tuesday. There is only one debate in that stretch. 

Jonathan Bernstein calls the race:

At this point, Romney essentially has the nomination wrapped up. Yes, people will point out that only a very small portion of delegates has been selected, but most of these contests are usually long over when the winner finally hits the mark that technically clinches it. Realistically, only some sort of external and utterly unexpected event could derail Romney now.

Douthat agrees that it's over:

There’s nobody waiting in the wings to help [Gingrich], no endorsement or donation that can change the fundamentals of the race. If he keeps going now – and there’s every reason to think he will – he’ll be pinning his hopes on a deus ex machina. Every realistic path leads only to defeat.

Josh Marshall wonders if Gingrich can keep going: