Firing the FBI director and trying to fire Mueller.
As Alan Dershowitz (Obama campaigner and Clinton voter), very liberal Harvard law professor is pointing out time after time, the president has the constitutional authority to terminate the employment of many people in the government, including the FBI director, so the president doing that, in and of itself, is a nothing burger, legally speaking. It only has "the appearance" of impropriety if you happen to believe a certain "narrative", i.e. in this case the mainstream media narrative that Trump is guilty of some unnamed crime (originally "collusion" which is not illegal, just like "extremely careless" is not illegal), which then morphed into "obstruction of justice", for which there is no evidence at all.
Tried? How do you know Trump "tried to fire Mueller"? What did he do that was "trying to fire Mueller", and how did it turn out that his effort to fire him failed and Mueller is still in place? "You're fired!" "No I'm not." Okay." As above, the president has the legal constitutional authority to fire a "special/independent counsel", so it would only be "damning" if you believe he did it in an effort to cover up something (a la Nixon)... and there is no evidence of that at all (the "information" in the "dossier" paid for by Clinton is fake).