It took Reid two conversations over the summer of 2011 to get Sanders to scrap the idea, according to multiple people who remember the incident, which has not been previously reported.

That summer, Sanders privately discussed a potential primary challenge to Obama with several people, including Patrick Leahy, his fellow Vermont senator. Leahy, alarmed, warned Jim Messina, Obama’s presidential reelection-campaign manager. Obama’s campaign team was “absolutely panicked” by Leahy’s report, Messina told me, since “every president who has gotten a real primary has lost a general [election].”

David Plouffe, another Obama strategist, confirmed Messina’s account, as did another person familiar with what happened. (A spokesman for Leahy did not comment when asked several times about his role in the incident.)

Messina called Reid, then the Senate majority leader, who had built a strong relationship with Sanders but was also fiercely defensive of Obama. What could you be thinking? Reid asked Sanders, according to multiple people who remember the conversations. You need to stop.