Mr. Stepien is expected to help beef up a political operation that has been a work in progress for several weeks. He had initially been contacted by the Trump campaign earlier this year as Mr. Christie’s campaign for president was sputtering. When the issue was revisited after Mr. Christie dropped out of the race, both the governor and Mr. Trump’s then-campaign manager, Corey Lewandowski, opposed hiring him, according to people briefed at the time.

Roger Stone, an informal adviser to Mr. Trump over many years, described the move as a positive one. ‘He’s a real professional and has the kind of contacts and experience the campaign could really use,” Mr. Stone said.

For Mr. Stepien, 38, it is a chance to resurrect what was once a promising career as a national political operative. He successfully ran both of Mr. Christie’s campaigns for governor. Mr. Christie had hired him as a consultant to the Republican Governor’s Association during his year as chairman of that group. In January 2014, Mr. Christie picked him to lead the New Jersey Republican party, calling Mr. Stepien “the best Republican operative in the country.” It was widely expected that he would run Mr. Christie’s presidential campaign.

Days after the appointment, Mr. Christie cut Mr. Stepien loose, when documents from a legislative subpoena revealed that a deputy chief of staff to the governor, Bridget Anne Kelly, had sent an email calling for “some traffic problems in Fort Lee,” the town gridlocked by the lane closings on the George Washington Bridge between the borough and Manhattan in September 2013. Prosecutors, who indicted Ms. Kelly and two other Christie allies in the lane closings, have said the tie-ups were retaliation against the Fort Lee mayor, Mark Sokolich, a Democrat, for declining to endorse Mr. Christie for re-election in 2013.

The governor gave little reason for why he dropped Mr. Stepien other than to say he had “lost confidence in Bill’s judgment” after other emails showed Mr. Stepien referring to the mayor of Fort Lee as “an idiot” — the kind of language Mr. Christie has been known to employ publicly.