The trial of Senator Robert Menendez was paused on Thursday after the judge announced that one of the senator’s co-defendants, Fred Daibes, a New Jersey real estate developer, had tested positive for Covid-19.

The judge, Sidney H. Stein, said it was “the expectation and the hope of the court” that the trial could resume Monday, given revised federal Covid guidelines and depending on how quickly Mr. Daibes begins to recover.

The Centers for Disease Control and Prevention no longer recommends that people isolate themselves if they have been free of fever for at least 24 hours without the aid of medications and if their symptoms are improving.

Mr. Daibes is on trial with the senator and another defendant in a sprawling bribery conspiracy case in which prosecutors say Mr. Menendez, 70, and his wife, Nadine Menendez, 57, accepted gold, cash and a luxury car in exchange for the senator’s agreeing to dispense political favors at home and abroad. Ms. Menendez’s trial was postponed until at least August because she is being treated for breast cancer. All four defendants have pleaded not guilty.

The delay comes near the end of the trial’s fifth week in Federal District Court where, one day earlier, Mr. Daibes, who is in his late 60s, could be heard coughing loudly throughout the proceeding. Then, on Thursday, Judge Stein said in court that Mr. Daibes’s lawyers had emailed him at 8:18 a.m., notifying him that their client was feeling “clearly much worse.”

“I don’t think we have much choice,” Judge Stein said Thursday morning. “I cannot proceed without a defendant being present.”