Alexandria Ocasio-Cortez believes Donald Trump has “already admitted” to impeachable offences.
The congresswoman discussed the president with journalists on Tuesday night, after Nancy Pelosi announced House Democrats were launching a formal impeachment inquiry.
“What is going on is that the president has committed several impeachable offences,” she said.
“What he has admitted to is already impeachable regardless of future developments. What he has already admitted to is an impeachable offence.”
Ms Pelosi, the speaker of the house, announced the inquiry after the president was accused of pressuring the leader of Ukraine to smear Democratic presidential hopeful Joe Biden.
On Tuesday, the president defended the contents of his 25 July conversation with Volodymr Zelenzsy, the president of Ukraine, in which he urged the European leader to investigate Mr Biden and his son.
The president has also confirmed that he withheld nearly $400m (£322m) in US aid to Ukraine but has denied doing so as leverage to force Mr Zelensky to initiate a damaging probe targeting the former vice-president.
Earlier this month it emerged that a whistleblower, believed to be a member of the US intelligence community, raised an “urgent concern” complaint about the president’s call.
The whistleblower has asked to testify before the House Intelligence Community, according to Adam Schiff, the panel’s chair.
Ms Ocasio-Cortez said she believed potential articles of impeachment could include several offences.
“What those articles will include could articulate several different offences including emoluments and asking [for] foreign assistance,” she said, adding that she deferred on the matter to Jerry Nadler, the chair of the House Judiciary Committee and other chairs of investigatory panels.
Democrats have repeatedly accused the president of violating the US constitution’s emoluments clause, which prohibits profiting from the presidency.
The impeachment inquiry poses the biggest threat so far to Mr Trump’s presidency.
The president has promised to release a transcript of the phone call with Mr Zelensky.