But the potential anti-semitism from the left was USED in a bad faith sort of way. It's highly likely that the torries won because they created a narrative of blatant anti-semitism from Corbyn and Labour as a whole. Was that justified?
As an American I try to stay in my own lane, but from how the election was explained to me by people in the UK it seems misleading to suggest Corbyn being called antisemitic was what made a dent. Labour waffling on Brexit because they needed voters on both sides to even have a chance seems a lot more relevant. Party dynamics are totally different too.
I think it's quite convenient to blame Labour's defeat on an attack campaign of anti-semitism against Corbyn, but I don't think it's entirely accurate. I think it probably played a much smaller part than other attacks (in bed with Iran, not patriotic, ad nauseam), and a tiny part compared to Labour's stance on Brexit (which was to sit on the fence in an attempt not to alienate their core voters in pro-Leave areas, despite Corbyn not being a big fan of the EU and almost certainly being pro-Leave himself). And that's before we even get to actual policy - Labour's manifesto was a bit of a mess and whilst it undoubtedly appeals to a significant proportion of the electorate, it didn't have enough support to get them into power and their refusal to compromise in order to get elected has cost them yet again. Centrism is a dirty word in Labour circles but if the alternative is not getting elected for a generation then perhaps more good can be done by moving further to the middle and actually getting elected? Especially with the Tories pivoting further to the right."Corbyn is an anti-Semitic leader" was the main thing they used to attack his character and destroy his popularity in the general populace for months and maybe even years, I can't even remember when that garbage started.
It absolutely was one of the biggest reasons he lost the election so badly.
And let's not forget although Corbyn was being smeared as being anti-Semitic, Boris Johnson was fucking around calling people picaninnies or referring to burqa-wearing women as "letterboxes". If you're trying to say that anti-semitism plays far more effectively than flat out racism then I'm not sure I agree.
And to get back to the OP's point, I can't see the parallels between attacking Corbyn on anti-semitism and doing the same to Bernie Sanders. Completely and utterly different scenarios.