Democrats can't handle the truth
There is a logical fallacy known as "argument from ignorance," which, briefly stated, is that the absence of evidence is not the same as evidence of absence. This phrase is being used by Adam Schiff, for example, in reference to the Mueller report on collusion with Russia by the Trump campaign.
He may spin it as insisting that Mueller's investigation did not exonerate Trump and so there must be more investigation of the charges. Exoneration basically requires that the accused could not possibly have committed the crime. Collusion isn't criminal, by the way.
The real goal is to maintain uncertainty in the public mind, as much as possible, and undermine President Trump and his efforts to lead the country. Democrats also hope to undermine his re-election and the down-ticket results for the GOP when Trump runs again. They want to keep that cloud over his head as long as possible.
I would think it's almost impossible to prove innocence in a case like this. Fortunately for the Dems champing at the bit to investigate him, they don't actually need or expect to find anything (maybe Waters does) — only to make his time in office as difficult as possible by preserving doubts in the minds of their hopeful base. Their continued failure will also be used to rally their voters in 2020.
Even though "exoneration" might overstate the results of the Mueller investigation, most people see absence of evidence in the context of the principle of innocent until proven guilty. Ergo, Trump is innocent of the non-crime of collusion. It is an assumption of innocence, not proof of innocence. And he is still president, despite the last nearly two years of opponents looking for some grounds to drag him out of the Oval Office, with as much humiliation as could be arranged. It must be horrible to be a Democrat, especially right now.
I think the Dems pushing the collusion narrative need a catchy name, like "collusion truthers" or maybe "Russia truthers." They'll still be investigating Trump after his second term, and after he someday dies, and probably arguing that Trump met with Putin on a grassy knoll to get his orders.
"Truthers"? Actually, I think it's obvious that they can't handle the truth.