The core purpose of the US constitution is to prevent tyranny. That’s why the framers of the constitution distributed power among the president, Congress and the judiciary. That’s why each of the three branches was designed to limit the powers of the other two.
In other words, the framers anticipated the possibility of a Donald Trump.
The framers also put in mechanisms to enforce the constitution against a president who tries to usurp the powers of the other branches of government. Article I Section 2 gives the House of Representatives the “sole power of impeachment”. Article I Section 3, gives the Senate the “sole power to try all impeachments”.
Donald Trump surely appears to be usurping the powers of the other branches. Under these circumstances, the constitution mandates that the House undertake an impeachment inquiry and present evidence to the Senate.