I think people would do well to let go of the idea that the parties as a whole have an ideology. George W. Bush and Donald Trump are not remotely the same. Bernie and Manchin are not the same.
Yes there's a good bit of overlap in the venn diagram of their supporters, but MAGA was functionally a hostile takeover of the GOP. The old guard was made to get on board or find a new job.
If Bernie had been elected in 2016 the outcome for the DNC would likely have been just as dramatic. If you want the Democratic party to be a leftist party, go make it one. We've literally seen a major party pushed to fascism. You know know change is possible.
While the RNC is not nearly as rigged as the DNC, They absolutely stacked the deck against Trump in 2016.
Mega donors poured money into his opposition throughout the primary. Hit pieces ran left and right. There was a "Never Trump" movement on the right during the general. He was staunchly opposed by some of the biggest names in right-wing media (Glen Beck and Mark Levin).
But he won anyway.
The left may never get a fair shake but it is incumbent upon us to win anyway. If you put a true leftist economic populist in the White House for just one term you will see dramatic change across the political landscape. But no one's just going to give it to you.
It took decades of planning and learning hard lessons to truly turn this country fascist. If right-wing extremists had thrown in the towel when Barry Goldwater got washed we wouldn't have Trump today.