Some 40 years after Richard Nixon resigned to avoid his likely impeachment by the House of Representatives, Washington is again talking impeachment. Members of Congress are denouncing the president’s contempt for constitutional law, while the president is raising money to fight the effort to remove him. But this time, the money pouring in would be just as well spent on defense against Bigfoot. Much of the debate has been more mythological than constitutional.

1. An impeachable offense is anything Congress says it is.

People pushing for President Obama’s impeachment have cited rationales ranging from the border crisis to Benghazi to Obamacare to thedismantling of “our constitutional republic, our national security, our electoral system, our economic strength, our rights and liberties.” In other words, anything goes. This echoes the characterization by Gerald Ford, who as House minority leader in 1970 made the ill-considered statement that “an impeachable offense is whatever a majority of the House of Representatives considers it to be at a given moment in history.” (This interpretation was, not surprisingly, part of a frivolous effort to impeach Supreme Court Justice William O. Douglas, whom Ford denounced for espousing “liberal opinions,” for defending the “filthy” Swedish film “I Am Curious (Yellow)” and for writing a magazine article that “praises the lusty, lurid, and risque along with the social protest of left-wing folk singers.”)

But Congress’s exclusive power to impeach does not license it to abuse that power, any more than the Supreme Court’s final say on laws gives it license to deliver arbitrary rulings. The framers carefully defined the grounds for impeachment as “treason, bribery, or other high crimes and misdemeanors” — language with British legal precedent. They clearly did not want removal of the president subject to congressional whim. Indeed, they rejected the addition of “maladministration” after James Madison cautioned that “so vague a term will be equivalent to a tenure during pleasure of the Senate.”

It is the standard that justifies the power, not the inverse, as Ford saw it. The fact that only two presidents have been impeached, and none have been removed, suggests that most members of Congress take the impeachment standard seriously...