Mr. Trump admits that about a week before the infamous phone call, he put nearly $400 million in congressionally appropriated military aid for Ukraine on hold. It was not released until Sept. 11, after news broke of a whistle-blower complaint about the Zelensky call, a complaint that the inspector general for the intelligence community found “credible” and “of urgent concern.” This strongly suggests there was no legitimate reason for the hold.

The Trump administration is expected to release a redacted version of the whistle-blower’s complaint this week , and Mr. Trump says he’ll supply a transcript of the Zelensky call. But an unredacted version of the complaint must be released , and since the transcripts Richard Nixon turned over to Congress were doctored, Congress should also demand a backup recording of the call. Assuming Mr. Trump refuses, Ukraine undoubtedly has a transcript and a recording. That recording could well be analogous to the Nixon White House tapes, which showed the president’s personal involvement in the cover-up, and Congress should request it immediately.

But even if it isn’t conclusive evidence of a shakedown, such a tape of the conversation is likely to show that Mr. Trump, like Mr. Nixon, used his presidential powers against political opponents. Among the grounds for the 1974 committee vote to approve three articles of imp eachment against Mr. Nixon was that he authorized I.R.S. audits of political opponents on his “enemies list” as well as an illegal wiretap on a White House staff member who worked for his chief presidential rival.

Like Mr. Nixon, Mr. Trump appears to have acted for his own personal political interests as opposed to a legitimate national interest. Mr. Nixon used his personal lawyer to pay hush money to the Watergate burglars. Mr. Trump used his personal attorney as the go-between with Mr. Zelensky, urging the Ukrainian president to work with Mr. Giuliani to investigate Mr. Biden.

Going through Mr. Giuliani says it all. If Mr. Trump had a shred of evidence against Mr. Biden or any legitimate governmental objective in view, he would have directed the Justice Department or State Department to work directly with Ukraine.