ROCHESTER, N.H. -- Democratic presidential candidate Joe Biden said Wednesday for the first time that U.S. President Donald Trump must be impeached for abusing the powers of his office to help his own reelection.

Biden made the remarks as part of a blistering 25-minute campaign speech in New Hampshire.

Biden said Trump is "shooting holes in the Constitution" by asking foreign powers to interfere in the 2020 election and then refusing to co-operate with a resulting House 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, Biden said. "He's not just testing us. He's laughing at us."

Trump quickly shot back in a tweet that it was "so pathetic" to see Biden calling for his impeachment. The president maintained that he had done nothing wrong.

Trump in a July phone call to Ukraine's leader had asked for "a favour" of investigating Biden and his son Hunter. House Democrats have begun impeachment proceedings against Trump based on a rough transcript of the July 25 telephone call.

Trump has repeated his unfounded assertions that Biden, as vice-president, used his office to protect his son from corruption investigations in Ukraine. Biden on Wednesday again called those theories "lies" and said Trump was peddling them because he fears facing Biden in a general election.

"We're not going to let Donald Trump pick the Democratic nominee for president," Biden said. "I'm not going to let him get away with it. He's picked a fight with the wrong guy."