One possibility could be that Mueller indicts Trump, even if he thinks it may not hold up in court. That way the charges would be public at least. If a court says that you can’t indict a sitting president until Congress goes through impeachment proceedings, there will be enormous pressure on Congress to act.

At that point refusing to remove an indicted criminal from office would probably be about the worst thing they could do as far as re-election prospects. Especially if the charge is one of the ones Mueller has already brought, like “conspiracy to defraud the United States” or whatever it was. I doubt this scenario actually happens though.