Go home, Joe Biden: you may not be drunk, but you are certainly acting like it.

On Sunday, during an event in New Hampshire, the Democratic presidential candidate’s gift of the gaffe struck again. Madison Moore, a 21-year-old student, politely asked Biden to explain why he came fourth in Iowa – a fair question, given he sells himself as the most “electable” candidate. Biden responded by asking Moore if she had been to a caucus before. She said yes; he called her a “lying, dog-faced pony soldier”.

Biden has used that odd phrase before; he appears to think it comes from a John Wayne film, although no one seems able to confirm this. But the phrase would not have been any more appropriate had a cowboy once said it. Moore laughed off the insult, but later told reporters it was “humiliating” to be called a liar on national TV. She added that she had not previously taken part in a caucus, but had nodded her head out of nervousness. “[Biden] has been performing extremely poorly in this race and the fact that he couldn’t just straight answer my question without bullying or intimidating just exacerbates that fact,” she said.

Biden and verbal blunders go together like Donald Trump and fake tan; there is nothing new about him coming out with an inappropriate turn of phrase. For the most part, his semantic stumbles have not been a liability. In the same way that Trump’s ineloquence makes him appear more authentic to some, Biden’s way with words can lend him a folksy charm.

This latest exchange felt different, though. It was not just a gaffe; it was a giveaway, a glimpse into how rattled Biden is. The fact that he resorted to an insult rather than introspection when challenged on his poor result in Iowa does not bode well. His campaign is not over yet, but it is increasingly unlikely that he will be the Democratic nominee.

When Biden announced his candidacy last year, he became the frontrunner immediately. He had name recognition and equity with three key segments of voter: older black Americans, moderate suburban white people and working-class people in the rust belt. He did not inspire the passion that Bernie Sanders did, but Sanders’ supporters skew young, and young people do not vote. Conventional wisdom said Biden was the most electable Democrat.

However, Trump has taught us that we can no longer rely on conventional wisdom. Biden may have looked more electable on paper, but he was clearly the Hillary Clinton of 2020 – and there was nothing to suggest it would turn out better this time. From the beginning, he did not seem to understand the desperate desire for change. He threw his energy into arguing he was the guy who could beat Trump, rather than taking the time to understand why Trump won. As John Wayne once said: “Tomorrow hopes we have learned something from yesterday.” The fact that Biden was feted as the Democrats’ best hope for so long shows that we have learned very little from 2016.

We should not write Biden off yet, but Iowa – and his reaction to those results – should lay to rest the idea that he is the most electable candidate. Indeed, he has dropped drastically in the polls. On Monday, Quinnipiac University released a national survey, conducted after Iowa, that showed Sanders in front, with 25% support. Biden is polling second, at 17%, with Michael Bloomberg at 15%.

There is a very real possibility that the fight for the nomination will be between Sanders and Bloomberg. If that proves to be the case, we will essentially have an independent and a Republican battling it out for the Democratic nomination. (Sanders is a lifelong independent, while Bloomberg was a Republican for six years during his stint as the mayor of New York.) I am not sure that is good news for the polarised Democratic party, but it is good news for Trump.

•Arwa Mahdawi is a Guardian columnist