On primary night, Mr. Clinton called Mr. Clyburn and they spoke for 50 minutes. “Let’s just say it wasn’t pleasant,” Mr. Clyburn says.

Mr. Clinton called Mr. Clyburn an expletive, say Democrats familiar with the exchange. Mr. Clyburn’s office would confirm only that the former president used “offensive” words. Some day soon, the congressman says, he’ll write about the incident. On Tuesday, he endorsed Mr. Obama for president.