If Donald Trump pressured his Ukrainian counterpart to investigate former US vice president Joe Biden then impeachment would be the only option, the US House Intelligence Committee chairman has warned.
Adam Schiff's comments came as the scandal surrounding an alleged complaint by a whistleblower rumbles on.
The complaint, which Mr Trump's administration has refused to let Congress see, is believed to relate to the Republican president's 25 July phone call with Ukrainian leader Volodymyr Zelenskiy.
US media reports claim Mr Trump repeatedly asked Mr Zelenskiy to investigate unsubstantiated charges that Mr Biden, when he was vice president, threatened to withhold US aid unless a prosecutor who was looking into a gas company in which his son was involved was fired.
Democrat Mr Schiff told CNN on Sunday: "If the president is essentially withholding military aid at the same time that he is trying to browbeat a foreign leader to do something illicit, to provide dirt on his opponent during a presidential campaign, then that (impeachment) may be the only remedy that is co-equal to the evil that conduct represents."
Members of congress are also looking into whether Mr Trump's lawyer Rudy Giuliani travelled to Ukraine to pressure the government to aid Mr Trump's re-election effort by investigating the activities of Mr Biden's son.
During an interview on CNN, Mr Giuliani was asked whether he had asked Ukraine to look into Mr Biden. He initially said: "No, actually I didn't," but seconds later he said: "Of course I did."
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