If Trump first Mueller it would be an admission that Trump engaged in impeachable offenses. It would have the same effect as the “Saturday night massacre” in 1973. Trump’s best strategy for re-election would be to focus on showing the anti-Trump were on a witch hunt, and to make the “own” anything that goes wrong (see, the economy tanked, because you wouldn’t listen me but went into “resistance” and abusing rights). Trump will want to argue that Mueller’s witch-hunt threatens anyone who expresses politically incorrect views. He can discredit the Democrats charge that Trump is a Russian “manchurian candidate” by asking the Congress for a massive appropriation to assist the military in opposing Russian expansion.
The above is what is making many Democrats concerned, since much of their caucus is in “reistance mode” with a heart set on impeachment, and they remembered what happened in 1998 when the Republicans decided Clinton impeachable and the country didn’t agree.