FOR THE GOOD OF THE PARTY:

[P]olls in which few voters consider him their second choice also predict he won't benefit when other candidates leave. So do polls that show his "favorable" ratings are considerably lower than his competitors'. So does his campaign style, which makes it likely that supporters of the other candidates have felt personally insulted by the reality TV star.

And so does the simple fact that Trump has dominated the media for months, making it likely that most voters who are likely to support him already do so, and those who don't have made a relatively firm decision not to.

Bush is out. John Kasich had a disappointing night as well. As a less conservative Midwesterner, Kasich didn't have to win in South Carolina, but finishing in a weak fifth place, with only about 8 percent of the vote, makes it clear that he received little help from his (mediocre) second-place finish in New Hampshire. He has little money, a hard-to-discern path to victory, and most likely a host of Republican insiders who will be pushing him to drop out.

Trump beat Rubio and Ted Cruz by about 10 percentage points each in South Carolina. Bush and Kasich combined for about 15 percent of the vote. There's no guarantee all of those go to Rubio, but it sure seems likely, all being things equal.

Rubio also seems likely to receive many more endorsements in future contested states, as well as plenty of money to fuel his campaign (including Florida money previously pledged to Bush).

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