For a young lawmaker straining to find his way amid personal tragedy, the Nixon fracas was an early test of Mr. Biden’s defer-to-convention instincts and a jarring introduction to the capital. After his election in 1972, Mr. Biden had spoken to Mr. Nixon in an uncomfortable, minute-long courtesy phone call from the president after Mr. Biden’s wife and young daughter were killed in a car crash that December. (“I understand you were on the Hill at the time, and your wife was just driving by herself,” Mr. Nixon said, stammering for a few beats, according to a recording. “The main part is, you can remember that she was there when you won a great victory.”)

In his floor speech on impeachment, labeled in the Congressional Record under “Fairness to the President,” Mr. Biden described himself as a harsh critic of Mr. Nixon, but he added that the president was due the “utmost presumption of innocence.”

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“In a day when equality is on everyone’s lips, I suggest that in practice, in extraordinary times involving our highest office of government, some people may be, in fact, more equal than others,” Mr. Biden said. “And in this case, I refer to the president. His official derelictions, if proved, and his subsequent ouster, should it occur, will have an impact on our institutions and upon ourselves as Americans, beyond that which it would have for any of us who are involved in such a proceeding.”

Moving through the Capitol, Mr. Biden seemed awed at the historical burden that greeted him. “We were walking down the hall and he said, ‘What a time for me to come into the Senate,’” recalled Rufus Edmisten, then the deputy chief counsel to the Senate Watergate committee. “‘I would come at a time like this.’”

In his 2007 memoir, “Promises to Keep,” Mr. Biden described a formative memory of a crucial meeting after Mr. Nixon asked Senator John Stennis, a conservative Democrat from Mississippi, to run interference for the White House by listening to the Watergate tapes and summarizing them for his colleagues, rather than insisting on their wide release.

“I remember what he said in the Democratic caucus that day: ‘I’ve thought long and hard on what my obligation is,’” Mr. Biden wrote, quoting Mr. Stennis, “‘and I’ve decided I am a Senate man. I am not the president’s man. Therefore, I will not listen to the tapes. I am a man of the Senate.’ I’m proud to say I am a Senate man, too.”

Mr. Kaufman said that it was not until August, days before Mr. Nixon’s resignation, that Mr. Biden became convinced that the president must go.