The skinny undergraduate from New York City was no one’s picture of a campus radical.

He wore an L.L. Bean jacket and a broad smile, sometimes broke into rap to get a rise out of friends and wrote self-deprecating columns for the college newspaper expounding on, among other things, the ethics of grinding on the dance floor.

But to friends and classmates who knew Zohran Mamdani at Bowdoin College in the early 2010s, there was no mistaking the intensity with which he took up his chosen cause: Palestinians’ struggle with Israel.

On a New England campus known more for athletics than activism, he founded a chapter of Students for Justice in Palestine long before the group became a polarizing national force, and led a campaign to persuade Bowdoin to join an academic boycott of Israel’s “oppressive occupation and racist policies.” (The college president said no.)

He was willing to engage with different perspectives — but only so far. When a spasm of violence shook the Middle East in 2012, classmates persuaded him to team up on a joint educational event with J Street U, a liberal pro-Israel group that supported a two-state solution.

To them, the session felt like a promising model for future collaboration. Attendance was strong. Everyone smiled for a photo.

Yet afterward, Mr. Mamdani politely shut the partnership down, according to his counterpart at J Street U, Judah Isseroff. It was nothing personal, he remembered Mr. Mamdani had explained, but Students for Justice in Palestine followed a policy of anti-normalization, meaning it would no longer be working with groups that supported Israel.