"...
The terms “high crimes and misdemeanors,” in their natural sense, embrace a very large field of actions. They are broad enough to cover all criminal misconduct of the President, — all acts of commission or omission forbidden by the Constitution and the laws. To the word “misdemeanor,” indeed, is naturally attached a yet broader signification, which would embrace personal character and behavior as well as the proprieties of official conduct. Nor was, nor is, there any just reason why it should be restricted in this direction; for, in establishing a permanent national government, to insure purity and dignity, to secure the confidence of its own people and command the respect of foreign powers, it is not unfit that civil officers, and most especially the highest of all, the head of the people, should be answerable for personal demeanor.
The term “misdemeanor” was likewise used to designate all legal offences lower than felonies, — all the minor transgressions, all public wrongs, not felonious in character. The common law punished whatever acts were productive of disturbance to the public peace, or tended to incite to the commission of crime, or to injure the health or morals of the people, — such as profanity, drunkenness, challenging to fight, soliciting to the commission of crime, carrying infection through the streets, — an endless variety of offences..."
From 1867.
Ergo, it is up to Congress to define his "misdemeanor".
