WASHINGTON — He sternly challenged one of President Trump’s nominees testifying in a Senate conference room. He discussed tax policy during lunch with members of the Democratic caucus. He displayed photographs of his trip to hurricane-ravaged Puerto Rico on the Senate floor. And he serenaded a smiling Senator Ben Cardin, a Democrat from Maryland, on his birthday.

Like a frenzied tourist eager to not miss anything, Senator Robert Menendez of New Jersey buzzed around the Capitol on Thursday. It was his first time back in Washington since his federal corruption trial began a month ago, and the Democratic senator was eager to cram in as much as he could.

“It’s been a packed day,” said Juan Pachon, the lone aide accompanying Mr. Menendez. He turned as he saw the senator make his way toward a stairway. Mr. Menendez is a member of the Senate Foreign Relations Committee, and a meeting was starting in 10 minutes. Mr. Pachon chased after him. “I haven’t even had breakfast.”

It was 2 p.m.

Instead of occupying his Senate office, Mr. Menendez has been occupying the defendant’s chair in a courtroom in Newark, where his slow-moving trial is stretching into its fifth week. He is accused of abusing his office to help his friend and co-defendant, Dr. Salomon Melgen, in exchange for luxury gifts and political contributions. Both men have denied the charges.