Haven't read the other replies, but a pattern of obstruction of justice, as noted by James Comey on repeated occasions. He only needs to be guilty of one instance of obstruction of justice to justify impeachment.

Off the top of my head:
-firing Comey while he was running an investigation into Trump's campaign.
-saying publicly that he fired Comey for the Russia investigation
-asking Comey to let an investigation on Michael Flynn's Russia ties go

Add in sharing codeword level intelligence shared with us from Israel and jeopardizing not only our own agents, but future information we would receive from allies. I think something like this, while not technically a crime because the president can legally declassify information, proves Trump is mentally unfit to be president, and there is a stipulation to remove such a president from office in the constitution.