But by Monday afternoon, a chill had gone out among people who were pushing Biden. They couldn’t figure out what had happened, but several sensed that everything had come to a screeching halt.

Then came Rep. Jim Clyburn of South Carolina, dousing cold water on Biden in an interview with The Huffington Post that landed late on Monday afternoon, taking those around both men by surprise.

A month ago, Clyburn caught a lot of attention for saying how open he was to a Biden candidacy and how anxious he was about Clinton’s email server. Then Monday afternoon, he said, “I would not advise him to get in.”

Frantic messages started going back and forth between Biden’s advisers and Clyburn’s. What was Clyburn trying to say? Did the Clinton campaign somehow put him up to it? Should the vice president call?

Biden advisers’ whole concept of a race assumed poor showings in Iowa and New Hampshire, which they’d chalk up in large part to his late entry. But he needed to win South Carolina — and to do that, he would need major support among the Democratic establishment and African-Americans. Clyburn was key to both.