Violation of his oath to enforce the laws of the land. That is the bedrock of the function of the executive branch of United States Government: to execute the laws. (hence the name of the branch)
We don't impeach people for real reasons, though. We need something sexy and appealing to the celeb loving crowd!
We've reduced impeachment to a cynical act of "ours is bigger than yours."
Therefore I think that it is almost always the wrong thing to do. In Clinton's case, for instance, the only legitimate issue was one of national security--did top secret discussions take place with "others" without necessary security clearance present. As you recall, nobody asked that question. They were only interested in DNA analysis of the stain on her dress.....
After all Franklin Roosevelt committed impeachable offenses in saving the world for Western representative governments. Abraham Lincoln committed impeachable offenses in saving the Union. But neither was impeached, much less convicted. There are plenty of other examples.
Impeachment is traditionally a petty act of tyranny by congress to destroy a president they do not like, not the reluctant removal of a criminal that it was meant to be. We don't need to go there.