Socialists who are trying to clear the 2020 Democrat primary field for Bernie are going with the argument that Bernie is the farthest left, there's no one to his left, he's the true left, etc., against the identity politics crowd who would place all sorts of other Democrats to Bernie's left, including Harris, Booker, and other liberals in good standing with the ID-pol camp.
They are quibbling over the definition of "left," attempting to wrest it away from meaning identity-oriented SJW and back toward meaning class-oriented populist. But words mean whatever the language community's consensus is at the time and place that they're being used. You can't re-define words and expect everyone else to go along with it.
And right now, in probably every country where there's a word for "left," it refers to identity politics rather than class politics. It means supporting a coalition of certain marginalized identity groups -- people of color, women, gays, foreigners, etc. (Other marginalized identities fall under "right" ID-pol -- founding stock natives, heterosexual men, etc.)
"Left" does not mean wanting to break up the cartels that run our society, wanting labor unions to play a stronger role in the economy, seeking government protection for certain industries, and so on.
Like it, or don't like it, but that's what it means.
It's no different for "right" or "conservative". Everyone understands that to refer to culture-war topics, not so much to deregulation, tax cuts, military expansion, and the rest of their material agenda.
The populists supporting Bernie should learn from Trump's successful populist insurgent campaign in 2016. He did not run as the "most conservative" or the "farthest right" -- indeed, his opponents continually decried him as "not a true conservative," not a real Republican. And that charge was true -- he was a moderate on culture-war topics.
More importantly, though, he did not emphasize these identitarian distractions one way or the other. Some Republicans going for the yuppie vote try to play the culture war game, only from the liberal side. But Trump avoided those topics altogether, sticking with trade and industrial policy, militarism / foreign policy, and immigration. Whenever the culture war came up, he waved his hands and then moved on to his familiar material issues.
As he put it (paraphrasing), "Folks, I do happen to be conservative, but who cares? Our country's a mess, and we don't have time to worry about who's conservative, liberal, or moderate."
Some of his supporters may have made earnest arguments about how he was actually the truest conservative of the field, and the true candidate of the right, based on some rejected usage of "conservative" and "right". They didn't feel wholesome voting for a moderate, and felt better if they could cast their choice as ideologically pure. But nobody else bought that argument.
The same must happen on the Bernie side for 2020. He should not bring up identity politics or intersectionality at all, except to briefly wave his hands while mispronouncing a shibboleth to prove to the normies that he isn't really "one of them" (psychotic SJWs).
Voters worried that Trump might be another Bible-thumping Republican were quickly relieved to hear him mispronounce "Two Corinthians". Only someone who's never been to church, and doesn't take religion seriously, would fuck that up so bad. Phew! Evangelicals like Ted Cruz would never have made such a mistake.
If Bernie were to botch a pronunciation, fumbling over "outer-sectionality", it would relieve the non-leftist voters that he has nothing to do with identity politics. After the neoliberal shills jumped on his mispronunciation in the broadcast media, social media, etc., that would only convince normies all the more that Bernie does not belong to their in-group. Phew! No way would we ever vote for someone who revered a concept like "intersectionality".
All this crazy hand-wringing over identity politics would give Bernie the perfect opportunity to, like Trump, say "Look, I do happen to side with you guys on social and cultural issues, but there are quite frankly far more dire fundamental problems facing this nation, so who cares about who's liberal, conservative, or moderate?"
Like Trump, he would not be taking the opposite side of his party in the culture war, but ignoring it altogether, and casting aspersions on those who fixate on it -- as Trump did to Cruz et al, who were more worried about trannies in bathrooms than whether the working class would continue to be able to support their families if the management relocated their factories to Mexico, or whether we should go into debt by another $7 trillion just so the generals can play Risk in the Middle East.
The fact that Bernie is not the "most left" of the 2020 Democrats -- e.g. on "Abolish the 2nd amendment" -- is a selling point for the general public, not something to obfuscate or apologize for. If you need to tell yourself he's the "most" something, to make yourself feel pure, tell yourself that he's the most populist or the most socialist. No one believes those terms refer to social-cultural issues, and nobody disagrees that Bernie is the most class / economics populist of the field, by far.
Abandon the pointless struggle to re-claim the definition of "left" from the long-victorious SJWs, and prioritize terms like populism or socialism, choosing appropriately for your audience. Symbolic battles are not supposed to matter to materialists -- so just let the SJWs have their stupid "left" word, and use that to marginalize them with a general audience in order to advance materialist populism.
"See, the SJWs themselves hate Bernie -- what more of an endorsement could you want, Independent / Moderate / disaffected voters?"