Greg Rosenbaum, the chairman of the National Jewish Democratic Council, raised such concerns this month at a private meeting between President Obama and Jewish groups. At the meeting, other Jewish leaders told Mr. Obama that his own rhetoric — framing the debate as a choice between diplomacy and war, and speaking of the money lining up against the deal — was only accelerating the corrosion.

“We are on the verge of fratricide in the Jewish community, and it has to stop,” said Mr. Rosenbaum, who spoke of Jews’ spurning organizational meetings, and even religious services, simply to avoid discussing Iran.

The animus is hard to miss. On his Facebook page, Mr. Nadler has been called a kapo: a Jew who collaborated with Nazis in the World War II death camps. One writer said he had “blood on his hands.” Another said he had “facilitated Obama’s holocaust.”

Dov Hikind, a New York State assemblyman, rented a double-decker bus this week; plastered the smiling face of Iran’s supreme leader, Ayatollah Ali Khamenei, on it; and parked it in front of Mr. Nadler’s office.