Manchester, N.H.

The big media story from the debate will be Marco Rubio’s confrontation with Chris Christie. But the larger picture might be about how well Donald Trump did.

Trump was relatively reserved. He wasn't bombastic. Or erratic. He was—by Trumpian standards—presidential? Okay, let's not get crazy. Trump wasn't able to stay in check for the full three hours—he couldn't help himself from lashing out at Cruz in his closing statement. And where candidates often lash out at the media when they get in trouble in a debate, Trump attacked the audience. The move was classic pro-wrestling—like Vince McMahon baiting the crowd. And it was so crazy that it kind of worked.

But here's the thing: Trump had, easily, his worst moment in any of the debates so far. Jeb Bush gutted him on eminent domain. Trump has never lost an exchange so decisively. Yet Trump wound up skating past that defeat.

Partly it was the dynamics of the debate that favored Trump. Aside from eminent domain, Bush focused on Rubio and Christie looked like he was on a kamikaze mission to destroy the Florida senator. Nobody else even glanced Trump's way.

And in spots, Trump was pretty good. His full-throated law-and-order defense of the police is likely to play well with Republican voters. When he talked about James Foley he—again, by Trumpian standards—was somber and serious.

Yet what really made Trump the winner is this: He entered the debate with a commanding lead in the New Hampshire polls and his only worry was a surging Rubio. Rubio got stopped cold—but by a guy who's polling at 5 percent here. The second-place slot got more muddled by the debate, not less. And that's good news for the front-runner.

The net effect of tonight's debate will probably be to preserve—or even extend—Trump's position on Tuesday night.