From early on, Trump gave his adversaries abundant excuses to declare him illegitimate, or illegitimate as far as their principles were concerned.
To skip ahead to the Fall and to imagine Hillary Clinton "re-born hard," refusing to debate the man who incited violence at his rallies; who crudely demeaned his opponents; who ran a campaign filled with stooges of a foreign power; who encouraged, celebrated, and exploited the actions of WikiLeaks; who ran a never-disavowed multi-year effort to de-legitimize the sitting president; and so on, is difficult precisely because many or most of those justifications had been present for months, and had already been "normalized." The refusal to debate would have been a clear statement, but the person able to make it in September-October would have been able to make it or its equivalent in January-February.
HRC's message in the end seemed to be that Trump was unfit and unacceptable, as Jeet Heer notes. Debating him contradicted or blunted that message, but so did a thousand other things she did and didn't do. She could also have chosen to let others make that argument, and to focus all of her strength on the "agenda" that Heer says she downplayed. Instead, she tried to do both - "It's an emergency! Here's my worker retraining proposal!" - in the political equivalent of dividing her forces in the face of the enemy.
In short, she tried to play the odds and play the game, just as Obama tried to play the odds as he calculated them, in the hope of getting by with a "normal" victory. Obama apparently was ready to go public on the Russian allegations by the Summer, for instance, but is said to have backed down when Mitch McConnell threatened to call it "politics." You don't have to take a position on the intrinsic importance of the Russian question to understand the contradiction in the Democrats' response to it, the same as the contradiction in relation to Trump: If it was a matter of the greatest significance, then there should not have been any backing down to McConnell. Obama should have treated the issue as an emergency, taken extraordinary measures, and alerted the public. That he did not treat the issue as an emergency then compromises his defenders now when they ask us to treat it that way now. I discussed this question in some detail at around the same time the Russian question was receiving its first major post-election public airing late last Fall (OAG #2).
The same goes in regard to dealing with Trump. Unfortunately for his opponents, the kind of candidate who could have refused to debate Trump and could have driven the point home and sustained it is not the kind of candidate that the Democrats were prepared to nominate. They don't understand how to be hard in that way. As I've been saying, that is the one or perhaps the only thing that Trump seems to understand: how to go on with passionate intensity vs those who lack all conviction. Instead of backing down to a McConnell like Obama, we can assume that in a similar situation Trump would have been blurting out the awful truth at the first opportunity and every subsequent one.
The point of this argument is not to imagine going back in time and persuading HRC to adopt a saving tactic at the last possible second. The point is to illustrated the problem that the anti-Trump coalition was not able to face last year, but may be forced to face eventually, or may be in the process of being forced to face. To beat Trumpism they will have to mimic it without, if possible, succumbing to it, just as to beat the fascist totalitarians, we once upon a time needed to get a lot more fascist-totalitarian than we had been (and lastingly).