Behind the congressional Democrats’ warning are the efforts of a Ukrainian lawmaker, Andriy Derkach, who was educated in a K.G.B.-backed school and was, until recently, closely aligned with a pro-Russian political faction in Ukraine. In May he released tapes of what he said were fragments of telephone conversations between Mr. Biden, then the vice president, and Ukraine’s president at the time, Petro O. Poroshenko.
The purported tapes did little to change the understanding of the elder Mr. Biden’s engagement with Ukraine’s leadership. Mr. Biden had publicly insisted that aid to Kyiv could be tied to the firing of a Ukrainian prosecutor, whom the United States and European nations had accused of corruption. So far, the accusations emanating from Mr. Derkach and others about Mr. Biden and his son, Hunter, have either been debunked or not been substantiated by any independent sources.
But the accusations, fueled by release of the tapes, led to more online conspiracy theories, which American intelligence officials have warned may well have their origins in Russia. Mr. Johnson has at various moments said he would subpoena evidence surrounding the dealings with Ukraine in the last years of the Obama administration, and The Washington Post reported this month that there was an effort among Ukrainian officials to pass the tapes to Mr. Johnson’s committee.
Mr. Trump has cheered his work. Pressure Mr. Trump put on Ukraine’s leaders last year to investigate some of the same issues ultimately led to his impeachment, and in addition to raising questions about Mr. Biden and his son, Mr. Johnson’s investigation has lent some senatorial legitimacy to Mr. Trump’s claims about the matter.
The investigation focuses on Hunter Biden’s work for a Ukrainian energy firm, Burisma Holdings, and whether his presence and lucrative paycheck were meant to improperly curry favor with the Obama administration. Politico first reported that the Democrats’ concerns were connected to Mr. Johnson’s work.
A spokesman for Mr. Johnson, Austin Altenburg, said the chairman and his staff had already requested briefings from F.B.I. officials, and he accused Democrats of hypocrisy for ignoring recently declassified documents that suggested the F.B.I. may have relied on potentially tainted information in its investigation of ties between Mr. Trump’s campaign and Russia.
The letter from the Democrats asked for the F.B.I. to share what it knows with all lawmakers, “given the seriousness and specificity of these threats.” The Democrats requested a briefing on the matter by the end of the month, when lawmakers are scheduled to leave Washington for several weeks.