The Impeachment Inquiry Is Fully Legitimate

In their efforts to trash the Democrats’ investigation, the president and his defenders are trashing the Constitution along with it.

In declaring the initiation of a formal impeachment inquiry against President Donald Trump, Speaker Nancy Pelosi was solemn: “We’re not here to call bluffs. We’re here to find the truth, to uphold the Constitution of the United States. This is not a game for us; this is deadly serious.”

In the weeks since, she has upheld that intention: Three different committees of the House of Representatives, with their Republican and...