But I don't agree with your initial assumption. I don't know too many left leaning folks who thought that Obama or Bill Clinton were the greatest presidents ever while they were in office.
It's the same in Australia. Once Howard was turfed out of his own seat, all of a sudden you couldn't find anybody who'd voted for him. "Who? What? Me? No, no, I was never taken in by that "children overboard" and "Iraq dossier" nonsense. And do you know he said he'd never ever bring in a GST, and then he did - and he still got back in? But nobody I know voted for him. Must have been electoral fraud or something. Maybe low-class poorly-educated people, all racists of course." But then with the recent string of PMs, Howard's starting to be rehabilitated in public opinion. And in ten years it'll go around again.
All these shenanigans are simply because nobody can bring themselves to say, "Well, I thought it was a good idea at the time, but I was wrong. Now I've changed my mind."
If Trump's actions cause the US stunning success, then in ten years everyone will say what a fine chap he was, except of course a few Democrats who say, "well it wasn't his success, really it was just a continuation of Obama's policies." And if Trump's actions cause the US stunning failure, then in ten years everyone will say how awful he was, "and if only he'd followed Obama's policies..."
Successes are always to my credit, and failures are always someone else's fault. I have always shared whatever progressive/conservative ideals are prominent right now, even twenty years ago, I was just really quiet about it. And Oceania has always been at war with Eastasia.