Mr. Trump has made the parallel easier to draw as he complains of a “witch hunt,” tramples ethical standards and shows no sign of the reasonable political behavior the nation sorely needs from him. Like Mr. Nixon, he regularly denounces real and imagined “enemies”; his White House is full of sycophantic assistants pressed to defend fantastic claims and policy distortions, as was Mr. Nixon’s. Like the Nixonites, Trump loyalists in the administration are clearly fearful of crossing their boss by attempting helpful criticism as the president plays daily with political fire.

Yet the differences are also worth noting. The public learned then that the Nixon team had plunged into rank criminality, discussing a million-dollar bribe for the burglars after they demanded ransom money for protecting the White House. And the political realities in Congress were of a different order. Back then, the Democrats enjoyed subpoena power through majority control of both houses so that, unlike now, they could freely investigate the scandal. Bipartisanship was such in 1973 that the Senate voted 77 to zero to create the select Watergate committee once the F.B.I. established the burglary’s connection to the Nixon re-election campaign.

In contrast, current Republicans revel in tooth-and-claw partisanship. Democrats remain a largely powerless minority as Republican leaders pretend they have no grave doubts about Mr. Trump, hoping to survive next year’s elections despite his unpopularity.

Most striking of all in the Nixon impeachment was the deus ex machina revealed unexpectedly in the Watergate hearings that gripped the nation on television and radio — Mr. Nixon’s supreme folly of crafting his conspiracies before the attentive microphones of a White House taping system to record his utterances for some imagined high place in history. When the Supreme Court ruled that the tapes were fair game for investigators, the nation finally grasped the extent of Mr. Nixon’s scheming. Denials from his “silent majority” base became pointless.

President Trump has hinted threateningly at the existence of tapes; so far it sounds like his characteristic bluffing. (Ironies abound. Mr. Trump’s complaints to the F.B.I. about damaging leaks recall that Deep Throat, the ultimate Watergate leaker to The Washington Post, was revealed to be W. Mark Felt, then the associate director of the F.B.I.)