After a month and a half of testimony from government witnesses, lawyers for Senator Robert Menendez this week are expected to begin rebutting the web of corruption charges facing New Jersey’s senior senator, once one of the most powerful Democrats in Washington.

Federal prosecutors in Manhattan are likely to wrap up their case against Mr. Menendez by Wednesday. Mr. Menendez’s lawyers will then begin to call witnesses; they have said they might call as many as four dozen.

Mr. Menendez, 70, has pleaded not guilty to charges that he doled out political favors to friends and foreign governments in exchange for bribes both eye-popping and mundane: hundreds of thousands of dollars in cash, gold bars, a Mercedes-Benz, Formula One race tickets, a reclining chair and an exercise machine.

He is charged with acting as an agent of a foreign government and is the first senator in American history to be indicted twice in separate bribery cases — facts that have infused the proceeding with a sober, precedent-setting tone.

One key unanswered question is whether the senator will testify in his own defense before jurors are asked to decide whether prosecutors from the U.S. attorney’s office in the Southern District of New York have proven their case beyond a reasonable doubt.

Mr. Menendez did not take the witness stand during his first bribery trial, and it seems unlikely now. The earlier case, in federal court in New Jersey, ended in a mistrial in 2017.