Mr. Sanders’s current standing might be surprising considering his circumstances as recently as a few months ago. He had a heart attack in October. He seemed stuck in third place in national polls, with around 15 percent support. Well-known candidates with just 15 percent support do not usually go on to win. A factional rival for support on the left, Elizabeth Warren, had steadily gained in the polls.

But the events of that same period opened a path for Mr. Sanders.

In one sense, Ms. Warren’s rise in the polls might have gone too well. Her standing began to falter around the time she matched Mr. Biden in national polls, which invited scrutiny from the news media and her opponents. Her position on “Medicare for all” funding and her weak standing in hypothetical polling matchups against the president offered avenues for attack. At the same time, Pete Buttigieg surged to the lead in Iowa; he and Ms. Warren promptly clashed, preventing either from solidifying a strong position in the state.

In most elections, conflict between these two idealistic liberal candidates would have been good news for the establishment favorite, Mr. Biden. But he never emerged as a classic establishment front-runner, someone with a daunting number of endorsements and superior resources. He found himself outspent, in some cases by a wide margin, by lesser-known rivals at exactly the point in the race when an establishment front-runner would have been expected to spend lots of money to overpower his rivals.

Mr. Biden’s weak elite support has done more than simply deprive him of the resources to capitalize on his opening in Iowa. It has led many elite supporters to seek an alternative, empowering rivals who have weakened Mr. Biden further. Some have gone to Mr. Buttigieg, who has raised far more money than Mr. Biden.

One elite supporter — Michael Bloomberg — has actually entered the race, and could sap Mr. Biden’s support on Super Tuesday on March 3. Well-timed endorsements at Mr. Biden’s expense could plausibly push another rival, Amy Klobuchar, into contention in Iowa.