Astonishingly, Mr. Trump’s attacks were successful in part because they were amplified by some of the G.O.P.’s most prominent leaders, like John McCain and Mitch McConnell. The chorus of opposition to Mr. Cruz didn’t end there; Terry Branstad, the six-term Iowa governor, said he wanted Mr. Cruz “defeated,” and the former presidential nominee Bob Dole said he preferred Mr. Trump.

As Nate Silver of FiveThirtyEight pointed out last week, the willingness of so many of the party’s most prominent officials to come out against Mr. Cruz — or even implicitly for Mr. Trump — is cause to rethink some core assumptions about whether the party will unconditionally oppose Mr. Trump.

The assumption that Mr. Trump would be uniformly and vigorously opposed by the party’s elite has always been the cornerstone of the case for why Mr. Trump was extremely unlikely to win the nomination. But that opposition simply hasn’t shown up, and, if anything, he is now benefiting from vocal opposition to Mr. Cruz. Over all, the possibility that the party will acquiesce to Mr. Trump looks a lot better than I would have thought even one month ago.

A Trump win in Iowa could make it more difficult for a mainstream candidate, like Marco Rubio, to mount a comeback later in the season — even if the establishment does intend to fight Mr. Trump after dispatching Mr. Cruz.