Though with his frequent appearances on national television, the two documentaries and books about him, Mr. Booker would seem to hardly need to tell anyone who he is, he has spent the last few days touting his New Jersey credentials.

Announcing his candidacy at the headquarters of Audible.com in Newark, he described his life as a journey from one of the state’s smallest suburbs to the mayoralty of its largest city. He talked about taking his prom date to the Jersey Shore — a regional tradition.

Standing next to former Senator Bill Bradley, another nationally celebrated New Jersey politician, Mr. Booker said he had “gotten to know a lot about criticism.”

“I’ve heard it,” he said. “Too much Twitter from the mayor, too much exposure. There’s not a criticism I haven’t heard over the years, I’ve heard it all. But there’s one thing that everyone has to admit about my life as a professional, from my days working in housing high rises here in Newark as a tenants’ rights attorney to my time as mayor, is that I do not run from challenges. I run toward them.”

He was imposing enough that one of the nation’s most popular governors, Chris Christie, announced he would spend $24 million to hold a special election in October rather than risk appearing on the ballot with Mr. Booker on the general election day three weeks later.

The three candidates opposing Mr. Booker in the primary have already signaled that they intend to make the primary a referendum on him.

(There are 700,000 more registered Democrats than Republicans in New Jersey, and the state’s best-known Republicans declined to run, so his toughest battle is most likely to be in August.)