There are several aspects of the ongoing Democratic addiction to the Trump-Russia collusion fixation that must be understood:

Anyone who has been paying attention knows that the Russians have interfered with elections in democratic countries for decades.  While their efforts will inevitably help one candidate at the expense of another, that is not their goal.  Rather, they seek to undercut confidence in the electoral process and thus undermine the cohesion of democratic societies.  Being their "choice" is not a compliment; it means they believe that your election will help them attain their goals.  It is very unlikely that 2016 was the first time they did so in the United States, but we have had no calls for an investigation of any role they may have played in the election of Barack Obama.  The techniques for interfering have undoubtedly improved over time; social media have become an important tool, and of course, Obama made social media an important part of his campaign.

Finding that the Russians interfered in the 2016 election and proving that they colluded with the Trump campaign are thus two very different things, but both Mueller and the Democrats have done much to conflate them.  The notion that Trump must prove that he didn't collude with the Russians violates not only the precept of "innocent until proven guilty," but also social science methodology.

There are two ways to prove a negative: first, demonstrate why the event could not occur, and second, examine every conceivable case and show that it did not.  Neither technique is available for the Trump-Russia collusion hypothesis, which is why the presumption of collusion has been reasserted with greater vehemence every time there is a failure to find any evidence supporting the notion.  The difficulty of proving a negative is one reason why we have the presumption of innocence at the heart of our legal system.

In a 1940 speech, Robert Jackson, then attorney general and a future justice of the Supreme Court, addressed the potential for abuse when a prosecutor "pick(s) a man" and then "put(s) investigators to work to pin some offense on him."  Any prosecutor "stands a fair chance of finding at least a technical violation of some act on the part of almost anyone"; in such a case, the real crime is "being unpopular with the predominant or governing group, being attached to the wrong political views, or being personally obnoxious."  Jackson didn't foresee using this technique to coerce potential witnesses into providing evidence.  That is what Mueller did.

There is also the problem of what the Russians might have been trying to accomplish.  Right up until the election, pollsters, pundits, and the journalistic profession were in near unanimous agreement that Hillary Clinton would be elected, probably in a walk.  Mark Cohen claimed that Trump expected to lose.  Unless the Russians had better polling data than anyone in America, they had to be expecting to have to deal with her as president.  Antagonizing her unnecessarily was not in their interest, so the likely objective of their hacking her computers was to find information that would be useful for blackmailing her.  It is curious that the FBI's probe into what might have been in the 30,000 deleted emails ended so close to the time Russian interference became an issue.  Was Hillary trying to discredit what they might have found by accusing them of colluding with Trump?

And while we're at it, could Hillary prove that her campaign had not received information from the FBI's surveillance of the Trump campaign, either from the FBI or someone working for the FBI?  Either of these would require an independent investigation, just as similar behavior did following the Watergate break-in.  Could the FBI prove that its surveillance had not been intended to assist Hillary's campaign?  And then we'd also have to investigate Barack Obama's involvement.  Could he prove he wasn't involved?

I can make other plausible allegations as well: can the Democrats prove that the Mueller investigation hasn't been a major success for the Russian goal of undermining faith in elections?  Would that prove that they are in collusion with the Russians?  This is the nature of a world in which making an allegation is sufficient and the accused is required to prove his innocence.  Do the Democrats really want to go there?

Maybe it's time to get back to the real business of government: governing.