“What we investigated was understood to be substantive and real,” said Gordon Freedman, who served as a staffer on Mr. Ervin’s committee. “We now live in an era where the truth has been eroded as a standard.”

Watergate investigators also had the benefit of the secret recordings made by Nixon in the Oval Office. By contrast, Mr. Trump did not tape his private conversations and he shredded White House documents while in office. Several of his former aides have defied subpoenas issued by the Jan. 6 committee, some justifying their intransigence through “executive privilege,” a phrase that entered the lexicon in the Nixon era. But none of Nixon’s top advisers invoked it and instead elected to testify before Mr. Ervin’s committee — a reflection of a Republican Party far different from the one today.

“It took a lot of guts for seven Republicans on the Judiciary Committee and three conservative Southern Democrats to do the right thing and vote to impeach Nixon,” said Elizabeth Holtzman, who 50 years after being elected to Congress and serving on the House Judiciary Committee is running for Congress again. “They didn’t do it to agree with me. They did it because they followed the truth. And they did it, really, because the American public forced them to.”