Senator J.D. Vance, an Ohio Republican, joined Donald J. Trump’s entourage in court on Monday as the prosecution’s star witness, Michael D. Cohen, the former president’s fixer-turned-nemesis, took the stand.
Mr. Vance and other supporters of Mr. Trump attacked Mr. Cohen and other participants in the trial in social media posts and a news conference. The judge overseeing the case has barred Mr. Trump from launching such broadsides.
“Every single person involved in this prosecution is practically a Democratic political operative,” Mr. Vance told reporters in Collect Pond Park, outside the courthouse. He added that it was a “disgrace” that Mr. Trump could not say so himself. Mr. Trump is barred from attacking a number of people involved in the case, including witnesses and prosecutors, other than the Manhattan district attorney, Alvin L. Bragg.
Mr. Vance’s presence could signal a new frontier for Mr. Trump’s testing of potential running mates. The former president has been encouraging vice-presidential contenders, including Mr. Vance, to grant interviews to cable networks to measure their performance, as well as inviting them to join him on the campaign trail and to attend fund-raisers.
Mr. Vance, who had been aggressively critical of Mr. Trump before running for office, has worked to repair that relationship, and is now one of his most vocal defenders in the Senate. His seat in court on Monday would seem to be the most visible show of that role.
During the past several weeks, Mr. Trump has often been joined in the courtroom by aides — and occasionally, ideological allies — who provide him support amid a trial that has exhumed mortifying details of his personal life.
On Monday, other Republicans rounding out Mr. Trump’s posse included Senator Tommy Tuberville of Alabama; Representative Nicole Malliotakis of New York; Attorney General Steve Marshall of Alabama and Attorney General Brenna Bird of Iowa.
“It is depressing. That courtroom is depressing,” Mr. Tuberville told the reporters outside the courthouse. “This is New York City, icon of our country, and we got a courtroom that’s the most depressing thing I’ve ever been in.”
Ms. Bird called what she saw in court “a travesty,” and Mr. Marshall agreed.
“I have never seen in my career a greater perversion of the criminal justice system,” he said.
Mr. Vance, attacking a witness as Mr. Trump cannot, dismissed the morning’s testimony from Mr. Cohen, who was Mr. Trump’s former fixer. Mr. Cohen described from the witness stand media deals meant to bury embarrassing stories. He discussed the 2016 double bombshells of the “Access Hollywood” tape, in which Mr. Trump discussed groping women, and the threat of Stormy Daniels, a porn actress, going public with her account of an affair with the candidate.
“I think his testimony is going to hurt with any reasonable juror,” Mr. Vance said, “and hopefully we have a few of those.”
Mr. Trump’s support group inside the courtroom added to the atmosphere of the trial by posting live updates on social media.
Among the Republican politicians was Eric Trump, one of Mr. Trump’s sons.
“I have never seen anything more rehearsed!” Eric Trump posted as Mr. Cohen testified.
Andrew Giuliani, another Trump ally and the son of his former lawyer Rudy Giuliani, sat in the back row of the courtroom, where he posted that Mr. Cohen was a “serial liar.” He also smeared the Manhattan district attorney’s office, which brought the case.
