Donald Trump has been "shooting holes in the Constitution" and should be impeached, Democratic presidential candidate Joe Biden has said.
It is the first time Mr Biden has called for such drastic measures to be taken against the president.
During a 25-minute campaign speech in New Hampshire, he said: "In full view of the world and the American people, Donald Trump has violated his oath of office, betrayed this nation and committed impeachable acts.
"To preserve our Constitution, our democracy, our basic integrity, he should be impeached."
Mr Biden was referring to claims the president asked foreign powers to interfere in the 2020 election and then refused to cooperate with a resulting House of Representatives impeachment inquiry.
"This is a president who has decided this nation doesn't have the tools, the power, the political will to punish bad behaviour," he added.
"He's not just testing us. He's laughing at us."
Mr Trump quickly shot back in a tweet that it was "so pathetic" to see Mr Biden calling for his impeachment.
Democrats launched their impeachment probe amid claims Mr Trump tried to pressure Ukraine's president Volodymyr Zelenskiy to investigate Mr Biden while at the same time withholding military aid.
Mr Biden's son was on the board of a Ukrainian energy firm, Burisma.
The Bidens have not been shown to have done anything wrong, while President Trump has called the impeachment inquiry a witch-hunt and dismissed claims he used military aid to influence Ukraine's president.
Mr Trump insists Mr Biden, as vice president under Barack Obama, used his office to protect his son from corruption investigations in Ukraine.
Mr Biden called those theories "lies" and said Mr Trump was peddling them because he fears facing him in an election.
"We're not going to let Donald Trump pick the Democratic nominee for president," Mr Biden said.
"I'm not going to let him get away with it. He's picked a fight with the wrong guy."