Under Article 2, Section 4 of the United States Constitution it is required,
"The President, Vice President and all civil Officers of the United States, shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors."
Treason definition:
"The crime of trying to overthrow your country's government or of helping your country's Enemies during War."