Consistency has never been Mr. Christie’s strong suit, and that showed in his endorsement on Friday. During the final days of his failed campaign, Mr. Christie said this of Mr. Trump: “We are not electing an entertainer in chief. Showmanship is fun, but it is not the kind of leadership that will truly change America.” On Friday he said of Mr. Trump: “He’s a good friend. He’s a strong and resolute leader and he is someone who is going to lead the Republican Party to victory in November.”

Mr. Christie’s good friend is the enemy of his enemy. Marco Rubio’s attack ads helped Mr. Christie along to defeat. He started to take his revenge on the debate stage in New Hampshire by savagely mocking Mr. Rubio for his robotic performance. Now Mr. Trump, embarrassed by Mr. Rubio’s slashing ridicule in Thursday night’s forum, is giving Mr. Christie a chance to finish the job. They lost no time about it on Friday, spinning up a medley of playground insults toward the Florida senator.

The bombastic governor may not fully realize that while he damaged Mr. Rubio in New Hampshire with his attack, it also showed voters who Mr. Christie really is. He may help his new best frenemy forever take Mr. Rubio down, but it’s near certain that Mr. Christie will further cement his national reputation as a venal, vindictive political bully in the process. His endorsement has already demonstrated that Mr. Christie will say anything in service of his ambition. Asked what he hopes to get in return, Mr. Christie played coy, saying that after his term ends in 2018, he wants to “go into private life and make money like Trump.”

After his performance on Friday, Mr. Christie had better hope that Mr. Trump, wherever he winds up, can find a little something for his new apprentice to do. If Mr. Trump should win the presidency, he might want to consider Mr. Christie for transportation secretary, since he already knows so much about traffic patterns on commuter bridges.