Democratic lawmakers warned President Trump Wednesday they’re preparing to subpoena White House officials to produce documents related to President Trump’s efforts to pressure the president of Ukraine to investigate Trump’s political rival Joe Biden and his son Hunter. House Intelligence Chair Adam Schiff, the California Democrat who’s leading the impeachment investigation, said he was “deeply concerned” that Secretary of State Mike Pompeo vowed to block current and former State Department officials from being questioned. And Schiff denounced President Trump’s attacks on the unnamed intelligence official who blew the whistle on President Trump’s interactions with Ukraine.

Rep. Adam Schiff: “The president wants to make this all about the whistleblower, and suggests people that come forward with evidence of his wrongdoing are somehow treasonous and should be treated as traitors and spies. This is a blatant effort to intimidate witnesses. It’s an incitement to violence.”

This comes after Secretary of State Mike Pompeo acknowledged he was on the July call when Trump spoke to Ukrainian President Volodymyr Zelensky. Meanwhile, The Washington Post reports Trump enlisted Vice President Mike Pence in his efforts to pressure Ukraine’s new leader. According to the Post, Trump ordered Pence to skip the inauguration of President Zelensky in May, and later had Pence tell the Ukrainian president that U.S. military aid was still being withheld, while demanding more aggressive action on corruption — which he would have understood included an investigation into the Bidens.