Democratic White House hopeful Joe Biden said Wednesday for the first time that Donald Trump should be impeached, arguing that the US president has "betrayed this nation" and violated his oath of office.

"To preserve our Constitution, our democracy, our basic integrity, he should be impeached," Biden, the former vice president whom Trump has repeatedly accused without evidence of corruption, told supporters at a rally in New Hampshire.

"He's shooting holes in the Constitution, and we can not let him get away with it," added Biden, the longtime favorite who is now neck and neck with Senator Elizabeth Warren in the race to take on Trump in the 2020 presidential election.

The remarks mark a significantly tougher stance by the Democrat who until recently has been the outright frontrunner in the race for the party's 2020 nomination.

Even after it was revealed that Trump asked Ukrainian President Volodymyr Zelensky to dig up dirt on Biden and his son Hunter, the senior Democrat had been more restrained than most other White House contenders.

But the gloves came off Wednesday, one day after the White House announced it would not cooperate with the impeachment investigation launched by House Democratic leaders last month.

"He believes the entire United States government can be corrupted into furthering his personal political needs," Biden said.

"He's even willing to hold Congress and congressionally appropriated aid to a foreign nation hostage to his personal political demands," he added, referring to accusations that Trump's call with Zelensky was an abuse of power because he allegedly tied Ukraine aid to investigating the Bidens.

Trump denies any quid pro quo, but has made no secret of calling on Ukraine and China to investigate whether the Bidens engaged in corruption.

Before Biden had finished speaking, Trump took to Twitter in a direct attack on him and his "failing campaign."

"So pathetic to see Sleepy Joe Biden, who with his son, Hunter, and to the detriment of the American Taxpayer, has ripped off at least two countries for millions of dollars, calling for my impeachment - and I did nothing wrong," Trump said.