They don't really have to have that, though. They just need to decide he's not fit to be president. I don't know why everyone is making such a BFD about the legal arguments.
The charge of high crimes and misdemeanors covers allegations of misconduct peculiar to officials, such as perjury of oath, abuse of authority, bribery, intimidation, misuse of assets, failure to supervise, dereliction of duty, unbecoming conduct, and refusal to obey a lawful order. Offenses by officials also include ordinary crimes, but perhaps with different standards of proof and punishment than for nonofficials, on the grounds that more is expected of officials by their oaths of office. -- Wikipedia