Donald Trump took another swipe at the FBI last week, calling it ‘disgraceful’. Some people thought he was trying to get the FBI to back off. More likely, he was insuring himself against anything the FBI finds against him.
No matter what the facts of what the FBI uncovers, Trump will say that it is untrue and just sour grapes from an agency that is out to get him because he had the balls to point out its shortcomings. If he can discredit anything the FBI (or Mueller) turns up, he will be way ahead in the court of public opinion. And impeachment is mostly a court of public opinion, which he knows very well.
As the man said, the best defense is a good offense. Discredit your potential accusers before they even start to present their case, and all of the sudden their facts become ‘facts’.