“This is the man that we need in this fight,” said Mr. Booker, whose 10-minute introduction reached such a crescendo that his voice grew hoarse. “This is the man who will help us to stand against the storm, this is the man that is ready to continue to defend the State of New Jersey.”

Nearly the entire 2,800-student population of Union City High School, excused from four periods of classes, filled the gymnasium, waving campaign fliers and bopping to the music of Marc Anthony and Luis Fonsi.

Mr. Menendez focused much of his speech on the youth in the crowd, recalling the nationwide protests for stronger gun laws and how the students at the School had their own protest. It was the youth, he said, that motivated him to run again.

“While some of you are not old enough to vote, I am not here to ask for your vote, I am here because I am your vote in the United States Senate,” he said.

Mr. Menendez’s path to the Democratic nomination is fairly clear. If successful, his likely Republican opponent in the general election will be Bob Hugin, a former pharmaceutical executive with ties to former Gov. Chris Christie. Mr. Hugin’s ability to tap his fortune to finance his campaign could make the contest expensive and possibly more competitive than expected.

The race could also turn nasty. At the rally, members of the state Republican Party handed out Monopoly-like cards with Mr. Menendez’s face on a cartoon prisoner and a photo of Mr. Menendez and Mr. Melgen vacationing together.