President Donald Trump is alleged to have pressured Ukraine in a way that would help him politically.
A deadline for the administration to decide whether to send a legal representative or witnesses to the House Judiciary Committee passed on Friday.
The committee is in the process of drawing up articles of impeachment against Mr Trump, leading to a vote in the House of Representatives before a trial in the Senate – possibly early in January.
An anonymous whistleblower complained to Congress in September about a July phone call by Mr Trump to the president of Ukraine, in which Mr Trump appeared to tie US military assistance to Ukraine launching investigations which could help him politically.
In return for those investigations, Democrats say Mr Trump offered two bargaining chips – $400m (£304m) of military aid that had already been allocated by Congress, and a White House meeting for President Volodymyr Zelensky.The first investigation Mr Trump wanted from Ukraine was into former Vice-President Joe Biden, his main Democratic challenger, and his son Hunter. Hunter Biden joined the board of a Ukrainian energy company when his father was President Obama’s deputy.
The second Trump demand was that Ukraine should try to corroborate a conspiracy theory that Ukraine, not Russia, had interfered in the last US presidential election.