In fact, it is right there article II, section 4
Withholding funds unless an investigation into the actions of a possible Presidential rival is carried out comes under the heading of bribery if proved.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.
At this point, the impeachment process is an investigation into alleged lawbreaking by Trump and perfectly constitutional.
It's doomed, obviously, as a partisan Senate acts as the jury and around 20 Republican Senators have to vote against their own self-interest in order to remove Trump from office.
What you are railing against is an investigation into alleged wrongdoings by
Trump.
To not investigate those allegations would be unconstitutional and lead to the President being above the law. Otherwise known as totalitarian government.