Keep in mind that the idea that a centrist has a better chance at winning isn't just because "This is what the pundits say". There have been studies done that show that moderate candidates do better than extremists at elections.
Trump didn't win because he was a centrist, he won because he presented himself as an anti-establishment outsider. A populist.
Yes, its true, being a 'populist' was a significant part of Trump's election strategy. (So was racism to be honest.) But, at the end of the day he still had to offer the electorate
something. And what he offered was a moderate position on many issues.
If he came out and said "I'm a populist, vote for me and I'll get America involved in more wars, privatize your social security, and send all gay people to work in the salt mines" I doubt he would have been successful.
I also think its a mistake to claim Trump's success was merely due to him being "an outsider". Voters are not some monolithic block... some may have voted for him because he was an outsider, some may have done so as a reflex "always vote republican", some may have been attracted specifically to his racism. And some may have liked his policies.
Sanders also has a populist message, and more importantly, real credibility on these matters. Compared to Trump's many failed promises, Bernie may appear very appealing.
Yes, he does have more credibility than Trump, and if people acted with logic and reason, Sanders would be a slam-dunk winner over Trump. But they don't. People are often naive and gullible, they sometimes listen to the wrong people and don't verify facts.
Except of course some of his policies seem to be rejected by that same working class.
Its all well and could to claim how those "working classes" will benefit under Sanders, but if those same people are rejecting your message right off the bat, you've certainly dug yourself into a hole early on.
This remains to be seen. Bernie doesn't have to win over dedicated MAGA CHUDs.
No, Sanders doesn't have to convert Trumpers to BernieBros. But he does have to make sure he 1) keeps the support of as many Democrats as possible, many of whom are not 'working class', but are suburbanites with white collar careers. 2) minimizes the number of Trump supporters who go to the polls.
In one poll, Sander's plan to eliminate all private health insurance was supported by just 30% of the population. Trump's current disapproval rating is around ~53%. That means roughly 20% of the population says "I don't like Trump, but I also don't like Sander's health care plan". That's a lot of voters to potentially piss off. If the Democrats start pushing that plan, they will lose votes among those who are happy with their current private insurance, AND they will probably end up increasing republican turnout, all in the name of 'stopping socialized medicine'.
https://www.cnn.com/2019/09/14/politics/poll-warren-sanders-health-insurance/index.html
Instead, he has the "I stole from my neighbors" scandal. And the "I'll ship radioactive waste through minority communities" scandal...And the "women enjoy rape don't they?" scandal ... And the "I went to a 'death-to-america rally' and all I got was this lousy T-shirt" scandal. And the "I honey-mooned in Moscow" scandal. And the "I called MYSELF a socialist" scandal.
I have a hard time imaging these mattering at all. These all sound pretty lame.
And there were probably some Clinton backers who thought "I can't imagine Clinton's emails mattering at all... it happened so long ago and there's never been a confirmed security breach".
How did that work out for them?
I really have a hard time imaging Republicans being effective smearing Bernie with anti-environment
The republicans don't have to convince a Democrat to vote for Trump based on environmental record.
They just need to convince the Democrat to not vote at all because "all politicians are the same".
Scandals only matter if they resonate with voters. Who knows what the future might bring. I will only point out that every attack so far on Bernie has failed spectacularly, and often backfired on those attacking.
As I pointed out before, Sanders hasn't really BEEN attacked. While the Republicans regularly condemned Clinton during the 2016 primaries, they were usually silent on Sanders. (And Clinton herself didn't get involved in anything low-level on Sanders to any significant degree.)
The 2020 primaries appear to be similar... The debates are mostly about policy, and so far Trump often appears to be more supportive of Sanders than attacking him. (Suggesting for example that Sanders is being 'robbed' of the nomination.)