The news that Chris Christie is endorsing Donald Trump shouldn’t come as much of a surprise. The men have long been friends, in that superficial New-York-power-brokery way, and when Christie dropped out of the race, a couple weeks ago, Trump let slip that he’d had a long phone call with the New Jersey governor the previous night. Trump is not a fellow for long phone calls (unless they’re on Morning Joe), so there had to be something worth discussing. An early and helpful endorsement like this puts Christie nicely in line for a nomination to head up the Department of Justice, should Trump win the White House. Even if Trump loses, I suspect he doesn’t forget a favor.

Trump needs Christie right now more than you may realize. He had at best a mediocre outing Thursday night on the debate stage. His strongest moment came when he voiced firm but non-fanatical support for Israel, stating that there’s zero hope of brokering a deal if you start by demonizing one of the interested parties—a statement that probably satisfied most voters and made Ted Cruz and Marco Rubio, who attacked him for it, look hysterical. Apart from that, Trump tended, when under fire, to resort to broad slogans and personal attacks, a reliable move in the past but one that’s increasingly threadbare as the cast of contestants narrows. Trump couldn’t hide an ignorance of policy specifics, and the excuse he provided for hiring foreign workers at his hotels—that you can’t find adequate help during the peak season—flies squarely in the face of the theoretical underpinnings of his immigration policy: plenty of American workers, after all, are available if you pay them enough.

Another threat to Trump has been the emboldening of Rubio. It started Thursday night, with Rubio mocking Trump for repeating himself, and continued the next day, with Rubio deciding to be among the first candidates to antagonize Trump. During a speech Friday, Rubio started to make fun of Trump for poor spelling, for hypocrisy, and, above all, for human frailty. Rubio pointed out that Trump had “one of those little sweat mustaches” backstage during the debate and kept applying makeup to his face. “Then he asked for a full-length mirror,” Rubio said. “Maybe to make sure his pants weren’t wet, I don’t know.”

Whether you like Rubio or not—he must have some fans out there—Trump has had this coming to him for a while. It was Trump’s innovation in this campaign to launch assaults on people’s very dignity, calling out Rubio for quirks like his own profuse sweating. The insults were hilarious, but the risk was akin to using a new tactic during battle. What if the other side uses it, too? Trump is lucky it took someone this long, but Rubio’s new edge is likely to get generous coverage.

The endorsement of Christie thus comes at a crucial moment, sustaining what appears to be Trump’s overall momentum. In only a few days, Trump could effectively close out the race, grabbing up wins in most of the states on Super Tuesday. The odds are in his favor. As amazingly fast as perceptions can change—what the writer Mickey Kaus has called the “Feiler Faster Thesis”—five days is very little time to slow down a rolling Trump-size snowball. Yes, anything can happen, and the assaults on him over the next few days will be savage. But, with Christie’s endorsement, Trump now has a great brawler in his corner, and one with plenty of incentive. The New Jersey governor can’t run for re-election in 2017, and he’s already made his disdain for Congress abundantly clear. Hitching his wagon to Trump’s star guarantees, if not a cabinet position, at least access to the billionaire’s tri-state business network.