Is US in constitutional crisis? That may not be most important question.

President Donald Trump’s pledge of non-cooperation with the impeachment inquiry of the Democratic-controlled House of Representatives may or may not be a “constitutional crisis.” But it is a stark new test of the nation’s founding legal document – perhaps the most important such trial since Richard Nixon declined to turn over the White House tapes. How it’s resolved could reset the balance of power between the executive and legislative branches of government in our polarized political age.

This test is only partly a legal matter. It’s also political – an...