There’s no doubt that Mr. Trump should have left the question of Joe Biden’s work in Ukraine to Attorney General Bill Barr, who’s conducting an investigation that touches on the intersection of Ukraine and the 2016 election. Nevertheless, Americans will ask why Mr. Biden was apparently blind to the appearance of impropriety when in 2015 and 2016 he pressed the Ukrainian government to step up anticorruption efforts while his son Hunter was working for Burisma, a Kiev-based holding company, owned by Mykola Zlochevsky.

Hunter Biden became connected to the company in 2014, when Ukrainian prosecutors were scrutinizing it for possible money laundering and official corruption. The latter stemmed from Mr. Zlochevsky’s time as a government minister in charge of doling out licenses for natural-gas extraction, many of which ended up going to companies controlled by Burisma. Neither Mr. Zlochevsky nor Burisma were ever charged with any crimes and they deny all wrongdoing.

Burisma put Hunter Biden and another American, Devon Archer (a friend of then-Secretary of State John Kerry), on its board. The Wall Street Journal reports the young Mr. Biden made as much as $50,000 a month during his five years with Burisma, despite having no background in natural gas or Ukraine.

Even so, the Obama administration let Vice President Biden take the lead in lecturing the Ukrainians about corruption. Growing knowledge of this situation, certain to emerge during any impeachment inquiry, could doom the senior Mr. Biden’s presidential hopes.