It is in the Constitution, bonehead

Impeaching Trump for Obstructing Congress Would Harm Checks and Balances

by Alan M. Dershowitz  

Written right in the damn thing, the same document that defines checks and balances. So, the answer is simple: Dershowitz is as stupid as that other guy, the one that sounds like an ice cream.

The requirement for impeachment is a majority vote of the House. Trump is impeached because most of the House wants it, as near as I can tell. Trump knows more about the process than any of the so called legal experts expounding.