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’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 a range of cable networks in order to measure their performance, as well as inviting them to join him on the campaign trail and to attend fund-raisers for his campaign.

But 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. Mr. Vance’s seat in court on Monday could also be chalked up simply as well-timed support for the former president.

Other Republicans rounding out Mr. Trump’s support group in the courtroom included Senator Tommy Tuberville of Alabama, who has a close relationship with the former president; Representative Nicole Malliotakis of New York; and Attorney General Brenna Bird of Iowa.

Seated near the Republican politicians were Eric Trump, Mr. Trump’s son, and Alina Habba, a lawyer and spokeswoman for the former president.