Treason , bribery , or other high crimes and misdemeanours.

Treason. Some of his supporters expressed admiration for Putin above their [b]own[/b] elected president during the last administration, so [b]they're[/b] traitors, too. And Trump trying to act as president even before his inauguration, without consulting the actual president, is treasonous. He would certainly think so if someone tries to usurp his position before HIS term is over.