Among the casualties of President Trump’s declaration of a national emergency to build his border wall is the reputation of the majority leader Mitch McConnell as a Senate institutionalist. The evidence of the last few days has confirmed, if there were still any doubt, that he is no such thing.

First, he helped prolong the longest government shutdown in American history by insisting that the Senate would act only with explicit approval from the president. Now Mr. McConnell has fully acquiesced in President Trump’s power grab by supporting an emergency declaration, which he opposed just weeks before, aimed at addressing a crisis that Senate Republicans know does not exist.

This display of obedience from the leader of a supposedly coequal branch of government is shocking only if you ever believed Mr. McConnell was an institutionalist. But his defining characteristic has always been his willingness to do anything and sacrifice any principle to amass power for himself. What separates him from the garden-variety politicians — what makes him a radical — are the lengths he is willing to go. Seeing this with clarity should help us grasp the danger to which he is subjecting the Senate — and, more important, our democracy.

The signs of Mr. McConnell’s malign influence were always there. Before he became a Senate leader, he dedicated himself to opening the floodgates for corporate money to flow into our political system. Mr. McConnell chased the McCain-Feingold campaign finance reform law all the way to the Supreme Court; the 2003 challenge to the law bears his name. Mr. McConnell lost that one, but his cause prevailed six years later when the Supreme Court overturned restrictions on corporate contributions in Citizens United.