Mr. Biden as vice president made several trips to Ukraine to press the government to root out widespread corruption. That included seeking the ouster of former prosecutor general Viktor Shokin, who had investigated a private Ukrainian gas company, Burisma Group, of which Hunter Biden was a board member. Mr. Giuliani has suggested Mr. Biden’s motivation was to protect his son, a lawyer who has been involved in several investment and consulting firms, although Mr. Shokin had already completed his investigation of Burisma Group before he left office.

Yuriy Lutsenko, Ukraine’s prosecutor general at the time, told Bloomberg News in May he had no evidence of wrongdoing by Mr. Biden or his son.

As for Mr. Biden, the excavation of Mr. Trump’s call means his interventions in Ukraine will also get a thorough vetting. As vice president, Mr. Biden threatened to withhold U.S. loan guarantees to a previous Ukraine government if a prosecutor investigating corruption wasn’t fired. The prosecutor was investigating, among other things, a Ukrainian natural gas company that hired Hunter Biden, Joe’s son, as a director and also retained Hunter’s law firm. The prosecutor was fired.

Joe Biden says his demand was in the U.S. national interest and had nothing to do with his son. That may be true, but it does appear to be the kind of a conflict of interest that Democrats accuse Mr. Trump of having all the time with less evidence.

Mr. Shokin had dragged his feet on those investigations, Western diplomats said, and effectively squashed one in London by failing to cooperate with U.K. authorities, who had frozen $23.5 million of Mr. Zlochevsky’s assets. In a speech in 2015, the U.S. ambassador to Ukraine, Geoffrey Pyatt, called the Ukrainian prosecutor “an obstacle” to anticorruption efforts, and mentioned the U.K. case, which he said led to the escape of illicit assets.

But Ukraine’s government was slow to fire Mr. Shokin, despite warnings from the International Monetary Fund and others that Western aid to the country would be cut off if it didn’t act. Mr. Biden, in one of his trips to Ukraine in 2016, pressured the government, telling them the U.S. would withhold $1 billion in loan guarantees. At an event at the Council on Foreign Relations two years later, Mr. Biden said he told Ukraine officials: “If the prosecutor is not fired you’re not getting the money. Well, son of a bitch. He got fired.”

Democrats will also face a choice of how to handle Mr. Biden’s conflict of interest when as vice president he demanded that Ukraine fire a prosecutor investigating corruption, including a company that hired Hunter Biden as a director. Will they defend Mr. Biden’s actions as appropriate but impeach Mr. Trump for asking Ukraine to investigate corruption and the Bidens? That would certainly be a fascinating political straddle.