Traveling aboard his campaign bus in early December, he told reporters, “If I were to win Iowa, I think it’s awful hard to stop me from winning the nomination.”

He added, “If I were to come in, like some of the polling has shown, 5 points behind, or 3 points behind, or 7 points behind, I think I survive.”

Last week, Mr. Biden suggested the outcome in Iowa could include four candidates within a few percentage points of each other, “essentially a tie and so everybody goes to the next stop.”

But he added, “if you come out of here and somebody’s 25 and you’re at 12, well then you’re done, in terms of Iowa.”

When the data was finally released, Mr. Biden’s deficit against Mr. Buttigieg approached that margin.