Democrats have demanded records from the president’s personal lawyer, the next step of an effort that could remove Donald Trump from office.
Rudy Giuliani has admitted to asking Ukrainian officials to investigate widely debunked corruption allegations against Democratic rival Joe Biden.
Lawmakers issued a subpoena for those communication records from Mr Giuliani.
The request is part of impeachment proceedings against Mr Trump, spurred by a whistleblower complaint.
The whistleblower, reportedly a CIA officer, revealed concerns among a number of White House officials that Mr Trump had attempted to pressure a foreign leader into investigating a Democratic political rival.
A rough transcript emerged last week of that call between Mr Trump and President Volodymyr Zelensky, revealing that Mr Trump urged Mr Zelensky to investigate the discredited allegations against Mr Biden, a 2020 Democratic frontrunner, and his son.
On Monday it emerged that the US secretary of state, Mike Pompeo, had listened in on the call, which is now at the centre of an impeachment inquiry against the president. Mr Pompeo was last week served with a subpoena by House Democrats.
And in a separate development on Monday, it emerged that President Trump had pushed another foreign leader for help connected to his domestic political situation. The Australian government confirmed that Mr Trump asked Prime Minister Scott Morrison to help assist the US attorney general, Bill Barr, with an investigation into the Russia inquiry that dogged Mr Trump’s first two years in office.
The White House reportedly restricted access to the call’s transcript to a small group of the president’s aides – as officials had done with the transcript of the controversial Ukraine call.
Why was Giuliani served with a subpoena?
Three chairmen of key House of Representatives committees – Intelligence, Foreign Affairs and Oversight – sent the subpoena notice to Mr Giuliani on Monday.
The subpoena was expected, as Democrats had earlier said they wanted to question Mr Giuliani, a central figure in the saga.
On the phone call, Mr Trump asked the Ukrainian president to co-ordinate with his lawyer on any inquiry into Mr Biden and son Hunter.