By unilaterally shuttering the government in order to get his way, Trump is saying he has the constitutional right not to execute the laws whenever it suits him. Farewell, Congress.
By directing the attorney general, the justice department, the FBI, and the secretary of the treasury to act in his own personal interest rather than in the interests of the American people, Trump is saying that a president can run the government on his own. Adios, constitution.
By unilaterally threatening to cut off trade with the second-largest economy in the world, Trump is saying he has sole authority to endanger the entire American economy. (Make no mistake: if he goes through with the threat, the US economy will go into a tailspin.)
By doing whatever he could to stop an investigation into Russian interference in the 2016 election, including firing the head of the FBI, Trump has told America it's OK for a president to obstruct justice. Goodbye, law.
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.
This may not be the practical political thing to do. But it is the right thing to do.