The demagogue may be boundlessly confident in his own skills and force of political personality, but he cannot succeed on those alone. He can thrive only in political conditions conducive to the effective practice of these dark arts, such as widespread distrust of institutions, a polarized polity and a fractured media environment in which it is possible to construct alternative pictures of social realities. Weak political parties now fall quickly into line with a demagogue who can bring intense pressure to bear on party officials and officeholders through his hold on “the base.” As we have seen with Mr. Trump, the demagogue can bully his party into being an instrument of his will, silencing or driving out dissenters. Republican officeholders know that Mr. Trump can take to Twitter or to Fox News or to the podium at rallies — or all of the above — to excoriate them for a weak will or disloyalty.

This is how the Republican Party has become Mr. Trump’s party. It is also why that party will not conceive of its role in impeachment as entailing a constitutional responsibility independent of the president’s political and personal interests. It has come to see those interests as indistinguishable from its own. In this way the constitutional defense of the case against Mr. Trump and the defense of his own interests become one and the same. As another fabled demagogue, Huey Long of Louisiana, famously announced: “I’m the Constitution around here now.”