They are trying to use the impeachment of William Belknap as their model. Belknap was President Grant’s Secretary of War.
Like a number of Grant’s administration Belknap was corrupt. Belknap was in charge of Indian trading posts and he was extracting kickbacks from a sutler that he appointed to run one of the posts.
Congress impeached him after he resigned. He was tried in the Senate and acquitted.
It was unconstitutional then, and it is unconstitutional now.
Of course, when has the Constitution stopped politicians???