These remarks are not merely one more instance of Mr. Trump’s failure to curb his impulses. This is his constitutional defense strategy. Mr. Trump’s White House counsel, informing the House of the president’s refusal to cooperate, declared that the impeachment process is unconstitutional and invalid — a “naked political strategy” — and advised that the president would not participate. It matters that the president’s lawyer, in a formal communication with the House, used rhetoric that might have been expected from the hardest-core political supporters. Once again, contrasts with past impeachments are illuminating. Bill Clinton’s White House counsel Charles Ruff testified before the House Judiciary Committee, pledging to “assist you in performing your constitutional duties.”

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.”

The implications for the constitutional impeachment process are dire. Until Mr. Trump, modern impeachment has ended with some generally positive assessment of its legacy. Nixon’s resignation appeared to indicate that serious charges could bring the parties together in defense of the rule of law. “The system worked” was a popular refrain, even if this was a somewhat idealized and oversimplified version of events. The Clinton impeachment suggested that the standards for an impeachable offense required a distinction between public misconduct and private morality, and Congress reclaimed its responsibility for impeachment from an independent counsel statute that was allowed to lapse.

The Trump impeachment is headed toward a very different summation. A demagogue can claim that Congress has forfeited the right to recognition of its impeachment power, then proceed to unleash a barrage of falsehoods and personal attacks to confuse the public, cow legislators and intimidate witnesses. So long as the demagogue’s party controls one of the two chambers of Congress, this strategy seems a sure bet.

When this is all over, we will not hear warm bipartisan praise for how “the system worked.” The lesson will be that, in the politics of the time, a demagogue who gets into the Oval Office is hard to get out.