House Intelligence Chairman Adam Schiff (D-Calif.) told reporters Thursday that President Trump's call for China to investigate Joe and Hunter Biden is "repugnant" and "ought to be condemned by every member" of Congress.

"A president of the United States encouraging a foreign nation to interfere again to help his campaign by investigating a rival is a fundamental breach of the presidential oath."

Why it matters: Schiff has been tasked with leading the impeachment investigation into Trump's alleged efforts to push the government of Ukraine to investigate Biden. As that investigation heats up, Trump on Thursday appeared to again call for a foreign power to interfere in the 2020 election — this time in public.

The big picture: Schiff spoke to reporters outside an ongoing deposition with former special envoy to Ukraine Kurt Volker, who resigned last week after being named in the whistleblower complaint.

Volker is expected to testify that he did not know that military aid to Ukraine had been frozen at the time of Trump's July 25 phone call with Ukrainian President Volodymyr Zelensky, but that he was aware of Rudy Giuliani's interest in having Ukraine investigate Biden, per the New York Times.

Volker reportedly told Giuliani that he believed allegations about the Bidens were baseless and has told colleagues he was not involved in the pressure campaign.

The whistleblower complaint notes that after the July phone call, Volker and U.S. Ambassador to the EU Gordon Sondland met with Ukrainian officials and provided them with advice on how to "navigate" Trump's demands.

Go deeper: Trump impeachment threat engulfs all corners of federal government