Shabbos Kestenbaum moved through the crowd that came to hear him speak early last month, shaking hands and trying to mask his shyness.

He looked collegiate in tortoiseshell glasses and a barely there ginger beard. But to the more than 300 people in attendance, who were mostly Jewish, he was a rock star. He was the kid who got into Harvard, made his parents proud and then exposed antisemitism from inside the ivory tower.

At the event, a scholarship ceremony in Lake Success, N.Y., for high school students fighting injustice, Mr. Kestenbaum would be speaking about antisemitism at Harvard, next to a survivor of genocide in Rwanda. “You’re the Harvard guy?” a rabbi asked him. “Don’t hold it against me,” Mr. Kestenbaum replied.

Over the last year and a half, Mr. Kestenbaum, a 26-year-old Orthodox Jew from the Bronx, has become the face of the Republican-led campaign against antisemitism in America’s top universities.

His determination to battle his alma mater in court helped build momentum for the Trump administration to challenge what it calls the “ideological capture” driving college campuses to the political left.