US Democrats are expected to unveil formal charges against President Donald Trump, a key move in the impeachment procedure against him.
A Democratic Party aide said the articles of impeachment would focus on abuse of power and obstruction of Congress.
Mr Trump is accused of withholding aid to Ukraine for domestic political reasons.
He has dismissed the process as a “witch hunt” and a “phony show trial”.
The expected announcement by the House of Representatives judiciary panel would lead to an impeachment vote by the full lower chamber.
If the articles are approved by the House – which is controlled by the Democrats – a trial in the Republican-held Senate will take place, possibly early in January.
The House Judiciary Committee is believed to have drawn up articles of impeachment, following two months of congressional hearings.
The impeachment process was launched after an anonymous whistleblower complained to Congress in September about a July phone call by Mr Trump to the president of Ukraine.
What is Mr Trump accused of?
In the phone call, 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.
Democrats say this pressure on a vulnerable US ally constitutes an abuse of power.
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. This theory has been widely debunked, and US intelligence agencies are unanimous in saying Moscow was behind the hacking of Democratic Party emails in 2016.
How does impeachment work?
Impeachment is the first part – the charges – of a two-stage political process by which Congress can remove a president from office.
If, following the hearings, 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 and remove the president – 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.
President Richard Nixon resigned before he could be impeached.