The Republican primary process doesn't have as many ways for party members to put their fingers on the scale.
Also they've misappropriated words like "leftist" and "socialist" so much that in my interaction with Trump supporters, at Trump events, I hear plenty of actual left and actual socialist policies presented as new ideas or attributed to Trump.
At a policy level, these people actually don't want neofascism, I've interacted with plenty. They really don't.
The Democrats tried to appeal to the hard right voter who found Trump icky. For that they were called socialist so and I know this is hard, people I spoke with associated the word socialism with the policies of Harris
In fact, the Democratic candidate has won the popular vote in all four of the most recent elections before this one (from 2008 - 2020, inclusive).
There was clearly a winning path with say, Bernie in 2016. The state by state Bernie/Trump matchup polling data consistently predicted a clear and decisive victory. Or, maybe Estes Kefauver 1952, or go back to the 40s and Gallup predicted Henry Wallace would have had a 1936 style landslide instead of the squeak they won with Truman.
As a hobby I've poured over archives of primaries, old newspapers, speeches, going back even to Hannibal Hamlin, Lincoln's first VP and how he got replaced.
I continue to claim that any actual left project (as opposed to whatever the propaganda industry is deciding to imagine the left is) would be far more successful under a Republican flag because they aren't as committed to the neoimperialist project.
That's why the Democrats had all the warring Republicans on their side this time.