Hunter Biden

Trump, due to leave office on Jan. 20, has privately been mulling whether to pressure the Justice Department to appoint a special counsel to look into Hunter Biden.

"I specifically asked prosecutors to check especially carefully those facts about Biden's alleged involvement. They answered that there was nothing of the kind," the former prosecutor general said.

The Senate Homeland Security and Governmental Affairs voted 8-6 in favor of the subpoena for documents related to work Biden did for Ukrainian energy company Burisma.

"There's no question but that the appearance of looking into Burisma and Hunter Biden appears political. And I think people are tired of these kind of political investigations," Romney said.

Vindman, who listened in on the conversation in his official capacity, would be the first whistleblower to have first-hand knowledge of the call.

Igor Fruman and Lev Parnas allegedly conspired to use foreign money to buy political influence.

The two married in Los Angeles last May after they had known each other less than a week.

Hunter Biden said he "did nothing wrong at all" in an interview with ABC News that aired on Tuesday, but conceded he may have provided an opening to attacks on his father.

"Hunter undertook these business activities independently," his attorney George Mesires said in a statement. "He did not believe it appropriate to discuss them with his father, nor did he."