The myth of Joe Biden’s ascendancy has begun to unravel, especially in Iowa. For most of the year, the former VP has dominated in national and state polling for the Democratic presidential nomination, but his polling has sputtered recently. Today’s New York Times/Siena poll knocks Biden all the way down to the fourth position — behind the mayor from South Bend:

Ms. Warren appears to have solidified her gains in the first voting state while Mr. Buttigieg has climbed quickly to catch up with Mr. Sanders and overtake Mr. Biden, the onetime front-runner. Ms. Warren is drawing support from 22 percent of likely caucusgoers, while Mr. Sanders is at 19 percent, followed by Mr. Buttigieg at 18 percent and Mr. Biden at 17 percent.

The survey is full of alarming signs for Mr. Biden, who entered the race in April at the top of the polls in Iowa and nationally. He is still in the lead in most national polls, but his comparatively weak position in the earliest primary and caucus states now presents a serious threat to his candidacy. And Mr. Biden’s unsteadiness appears to have opened a path in the race for other Democrats closer to the political middle, particularly Mr. Buttigieg.

The poll reveals a race in flux but not in disarray, framed by a stark debate about the direction of the Democratic Party and by a degree of fluidity arising from Mr. Biden’s travails. In the early states, at least, the former vice president appears to be buckling on one side to the expansive populism of Ms. Warren and Mr. Sanders, and on the other to Mr. Buttigieg’s calls for generational change.

Biden’s crash in Iowa isn’t limited to this series. As late as mid-September, Biden had a 28.5% RCP average in state polling, with Warren and Sanders both ten points in his rear-view mirror. Here’s what’s happened over the last six weeks, with Biden as the green line:

That is a picture of a campaign falling off a cliff. That matters more for Biden, since his campaign was built on both a sense of inevitability and on an assumption that he’s the most electable candidate in the field. Both of those assumptions have dissipated, especially inevitability.

That decline isn’t just evident in Iowa, either. Nationally, Biden’s still vacillating between 25-30%, and Warren’s experiencing a more serious decline from 26.8% on October 9 to 21.3% today. In New Hampshire, though, Biden went from a summer high of almost 35% to 21% today, in a virtual tie with Sanders for second place and Warren ahead at 25%. Biden still has a wide lead in more conservative South Carolina, but he’s dropped a point behind Warren in California, whose delegates will be up for grabs much earlier in this cycle.

What has happened to produce this dramatic decline? Moments like this don’t help:

I’m pretty sure that Biden means the Paris Climate Accord, which puts this among Biden’s more benign gaffes. The problem with this one is that it accentuates Biden’s ancient reach in Beltway politics and makes him look more like a relic than Elizabeth Warren, whose age isn’t that far off from Biden’s but whose relevance is head and shoulders higher to younger voters. Take a look at this graphic from the NYT/Siena age demos to see how Biden’s playing among younger voters:

Hoo boy. Voters below age 45 are running away from Biden, even if they are mainly running to the two other septuagenarians in the race. Biden’s dominating among older voters, but trailing Andrew Yang in the younger two demos is not a good look for electability.

Biden has proven himself to be a largely inept presidential candidate in two previous attempts at the Democratic nomination. In those two campaigns, Biden didn’t have the advantages he does in this one — familiarity, the afterglow of the Barack Obama administration, and a Republican incumbent that seems best suited for Biden’s supposed reach to working-class suburban voters. It looks like even with those advantages, Biden’s still proving to be inept — and Democratic voters are finally noticing it.