I mean, this is true of anyone who adheres to a party. This isn't "leftist" as much as it is "democrat, republican, and liberal" party affiliation behavior. You'll also notice that, despite the statistical probability of Trump having the 10% drop down rate exclusive to himself in the election across 6 swing states (which he did have) being so low that it is statistically equivalent to winning the Powerball 29 times consecutively, the focus is the "epstein files." Like, yeah, that does matter but let's be honest - they have had the files for 10 months to themselves. We aren't going to ever see them in any sort of honest fashion anymore and Democrats know this. They know the files have spent too long in the hands of pedos for anything they do release to be trustworthy and they will have obviously scrubbed Trump from the list. That said, Clinton is almost certainly on it too.
What is much more prevalent IMHO is that the chances Trump won without someone or something influencing every swing state in his favor is the same as winning the powerball 29 times back to back without cheating. I can do the math for you through a proper standard deviation and Z-Score plot if you like but suffice to say - those are the odds. The odds Trump did not cheat are the same that you win the powerball 29 times in a row without cheating. That's definitively a bigger bullet IMHO and it's backable by raw data and mathematics rather than "lemme see them files, Donny."
All of this to say that your point has validity but it's misdirected. It's not the "leftists" doing this. It's established parties doing it.