US Democrats have initiated a formal impeachment investigation against President Donald Trump over allegations that he sought political help from Ukraine.
Announcement of the move Top Democrat Nancy Pelosi told the president "must be held accountable".
Trump denied the inappropriateness and called the efforts "garbage".
Although the Democrats strongly support the impeachment, it is unlikely that the investigation will pass the Republican-controlled Senate.
What is this dispute?
US intelligence officials reportedly complained last week to a government guard about Trump's interactions with a foreign leader, who was later exposed as President of Ukraine, Volodymyr Zelensky.
The whistleblower's complaint, which was considered "urgent" and credible by the Inspector General, was demanded by the Democrats, but the White House and the Ministry of Justice refused to submit it.
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Mr. Trump admitted that he had discussed Joe Biden with Mr. Zelensky, but said he was just trying to get Europe to step up aid by threatening to withhold military aid.
What did Ms. Pelosi say?
Ms. Pelosi said that Mr. Trump "committed a violation of the law" and called his actions "a violation of his constitutional obligations".
"This week, the President has admitted asking the President of Ukraine to take action that benefits him politically," she said, adding, "The President must be held accountable."
She has so far resisted calls from her liberal base to remove the Republican president from office, as such an effort could reinforce his support.
Mr Biden has also endorsed impeachment unless the US President approves the investigation.
Mr. Trump's charges "would be a tragedy," said Mr. Biden. "But a tragedy of his work." The former vice president is the current leader in the 2020 elections against Trump.
How did Trump react?
In a series of tweets, Trump said Democrats would have to "purposely ruin and humiliate" his trip to the UN "with more topical news about witch-hunt garbage".
"You have not even seen the transcript of the summons, a total witch hunt!" he added.