Two of Sanders’ top advisers — pollster Ben Tulchin and speechwriter David Sirota — told Sanders that he should pointedly take on Biden at the Feb. 25 debate in South Carolina, which the former vice president saw as his firewall state. They also said Sanders should use billionaire Mike Bloomberg as a foil, and attack him over his record of supporting stop-and-frisk, in order to deflect from the onslaught he would no doubt face as the newly anointed frontrunner.

The stakes were high: In an email to senior staff, Tulchin said that moderate voters were beginning to unify behind Biden and that the consolidation would intensify if he won South Carolina, according to people familiar with the message.

But on the stage that night, Sanders didn’t take his aides’ advice. Instead, he largely gave Biden a pass, bashed Bloomberg sometimes — but not over stop-and-frisk — and mostly stuck to his standard talking points. It wasn’t the first nor the last time Sanders eschewed his staffers’ suggestions to be more aggressive with his top rival. Their warnings proved prescient: Biden went on to sweep the day in South Carolina, unify moderates, and then carry Super Tuesday.