Donald Trump is expected to become only the third US president to be impeached later.
Democrats in the House of Representatives are set to vote to put him on trial - although the Republican-controlled Senate is unlikely to convict him.
Two articles of impeachment have been prepared.
One accuses Mr Trump of abuse of power, claiming he used the powers of his office to 'solicit the interference' of the Ukrainian government in the 2020 presidential election.
Democrats allege the US President pressured Ukrainian leader Volodymyr Zelensky to launch investigations that would damage Trump's own political rival Joe Biden, in return for releasing US military aid to Ukraine and setting a White House meeting between the two men.
The second article says Mr Trump obstructed Congress by ordering officials to not comply with the impeachment investigation.
In a furious letter to House leader Nancy Pelosi yesterday, Mr Trump accused Democrats of an 'unprecedented and unconstitutional abuse of power' during the impeachment process.
He argued: "You have cheapened the importance of the very ugly word, impeachment!
"By proceeding with your invalid impeachment, you are violating your oaths of office, you are breaking your allegiance to the Constitution, and you are declaring open war on American Democracy.
"It is a terrible thing you are doing, but you will have to live with it, not I!"
Mr Trump - who has repeatedly denied any wrongdoing - reiterated his insistence that his controversial phone call with Mr Zelensky was "perfect".
He also alleged Democrats are "falsely accusing me of doing what Joe Biden has admitted he actually did".
Impeachment vote
Today will see the House spend at least six hours debating the articles of impeachment ahead of a vote.
The Washington Post reports that its own analysis shows that House Democratic leaders now have enough votes to support impeachment.
A Senate trial would then likely take place in the New Year.
However, a two-thirds super majority of senators is needed to formally convict a president.
That is unlikely to happen in the currently Republican-controlled Senate, meaning Mr Trump would remain in office.
Two US presidents have previously been impeached by the House of Representatives - Andrew Johnson in 1868 and Bill Clinton in 1998.
However, neither was ultimately convicted by the Senate.
Richard Nixon also faced impeachment hearings, but resigned before the process got underway.
