The US House of Representatives is set to vote on how the impeachment inquiry into President Donald Trump should proceed.
The resolution to be voted on sets out a more public phase of the process and is not a ballot on whether or not to impeach the president.
The measure also sets out the rights Mr Trump's lawyers would have.
This will be the first formal test of support for the inquiry in the Democratic-controlled House.
President Trump is accused of trying to pressure Ukraine into investigating unsubstantiated corruption claims against his political rival, Joe Biden, and his son who worked with Ukrainian gas company Burisma.
Mr Trump denies wrongdoing and calls the impeachment inquiry a "witch hunt".
Republicans have criticised Democrats for the closed hearings up to this point, in which Republican lawmakers have also taken part. But Democrats insist they were needed to gather evidence ahead of the public stage of the inquiry, and deny allegations they have been secretive.
The debate began at 09:00 (13:00 GMT) and a vote is expected later on Thursday. Democrats, who have 234 of the 435 seats in the House, need a simple majority to approve the resolution.
What does the resolution say?
Earlier this week, Democrats in the House released an eight-page document setting out a two-stage process for the next phase of the inquiry.
In the first stage, the House Intelligence Committee would continue its investigations and hold public hearings. It would have the right to make public transcripts of depositions taken in private.
In the second stage, a public report on the findings would be sent to the House Judiciary Committee which would conduct its own proceedings and report on "such resolutions, articles of impeachment, or other recommendations as it deems proper".
President Trump's lawyers would be allowed to take part in the Judiciary Committee stage. Republicans on the committees would be able to subpoena documents or witnesses although they could still be blocked as both committees are controlled by Democrats.
Democrats are planning public hearings to be held in coming weeks and are considering the possibility of drafting articles of impeachment against the president.
What's behind the impeachment inquiry?
At the heart of the impeachment inquiry, which was announced by House Speaker Nancy Pelosi last month, is a phone call on 25 July between Mr Trump and Ukrainian President Volodymyr Zelensky.
A rough transcript of the call shows Mr Trump asking Mr Zelensky to investigate unsubstantiated corruption claims against Joe Biden, a leading Democratic candidate to challenge Mr Trump for the White House next year, and his son, Hunter.
The Trump-Zelensky phone call was the subject of a complaint by a whistleblower, whose identity has been closely protected by lawmakers. On Wednesday, a conservative author named a CIA officer as the whistleblower.
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 Richard Nixon resigned before he could be impeached
What else is happening?
Also on Thursday, Tim Morrison, who served as top adviser on Russia and Europe at the National Security Council, is expected to testify. He was one of the officials authorised to listen in on Mr Trump's call with the president of Ukraine, and unexpectedly resigned from his position on Wednesday.
Meanwhile, House investigators invited former US National Security Adviser John Bolton to testify on 7 November. His lawyer, Charles Cooper, told US media his client was not willing to appear voluntarily but that he was ready to accept a subpoena - or legal summons to compel testimony - on Mr Bolton's behalf.
US media say Mr Bolton was alarmed by a back-channel operation in Ukraine carried out by President Trump's private lawyer, Rudy Giuliani, who acknowledged last month having asked Ukraine to investigate Mr Biden.