Democrats have demanded records from the president’s personal lawyer, Rudy Giuliani, in the next step of an effort that could remove Mr Trump from office.
Mr Giuliani has admitted to asking Ukraine to investigate corruption allegations – widely debunked – against former vice-president Joe Biden.
Lawmakers issued a subpoena for those communication records from Mr Giuliani.
A phone call between President Donald Trump and the president of Ukraine was flagged by a whistleblower.
A rough transcript emerged last week of that call between Mr Trump and President Volodymyr Zelensky.
The transcript shows the US president urged him to investigate the discredited allegations against Mr Biden, a 2020 Democratic frontrunner, and his son.
The call is now at the centre of an impeachment inquiry by Democrats – an effort that could see Mr Trump expelled from office, but doing so would require members of his Republican party to turn against him.
What just happened?
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.
During a 19 September appearance on CNN, Mr Giuliani confirmed he asked Ukraine to “look into the allegations that related to my client, which tangentially involved Joe Biden in a massive bribery scheme”.
In their subpoena letter, the chairmen said: “In addition to this stark admission, you stated more recently that you are in possession of evidence – in the form of text messages, phone records, and other communications – indicating that you were not acting alone and that other Trump Administration officials may have been involved in this scheme.”
The chairmen have demanded all relevant communications be submitted by 15 October.
Mr Giuliani and the White House have not yet responded to the subpoena, though on Sunday, Mr Giuliani told ABC News he “wouldn’t co-operate” with Mr Schiff.
What is the impeachment inquiry about?
An intelligence community whistleblower filed a complaint last month, saying he had an “urgent concern” that Mr Trump had used his office to “solicit interference from a foreign country” in the upcoming 2020 presidential election.
Democrats launched an impeachment inquiry on Tuesday focusing on the call flagged in this complaint, where Mr Trump spoke with the Ukrainian leader about the Biden allegations.
Mr Trump’s political rivals have accused him of improperly seeking foreign help to try and smear Mr Biden and using military aid as a bargaining tool.
Ukrainian ex-prosecutor general Yuriy Lutsenko has told the BBC that there was no reason to investigate Mr Biden or his son Hunter Biden, who had worked for a Ukrainian energy firm.
Congress is currently in recess, but Democrats plan on remaining in Washington over the break to move “expeditiously” on with impeachment proceedings and additional subpoenas.
Quick facts on impeachment
- Impeachment is the first part – the charges – of a two-stage political process by which Congress can remove a president from office
- If the House of Representatives votes to pass articles of impeachment, the Senate is forced to hold a trial
- A Senate vote requires a two-thirds majority to convict – unlikely in this case, given that Mr Trump’s party controls the chamber
- Only two US presidents in history – Bill Clinton and Andrew Johnson – have been impeached but neither was convicted and removed
- President Nixon resigned before he could have been impeached
Trump impeachment: Lawyer Rudy Giuliani subpoenaed for documents