Democratic White House hopeful Joe Biden said Donald Trump betrayed the US.

Democratic White House hopeful Joe Biden called Wednesday for President Donald Trump's impeachment, saying he'd "betrayed" the United States.

Trump, however, showed no sign of buckling under pressure from the impeachment probe into his alleged bid to damage Biden by pressuring Ukraine to investigate the former vice president for corruption.

"To preserve our constitution, our democracy, our basic integrity, he should be impeached," Biden told supporters at a rally in New Hampshire, adding his voice to that of other Democratic contenders who want to see Trump impeached.

"He's shooting holes in the constitution, and we cannot let him get away with it," added Biden.

Trump is working hard to squash the impeachment probe before it can gather pace.

In an eight-page letter signed Tuesday by the White House counsel, the administration bluntly announced it would not cooperate with the Democratic-led House of Representatives, calling its impeachment push illegitimate.

Democrats responded by accusing Trump of stonewalling and obstruction.

"No one is above the law, not even President Trump," the Democratic majority leader in the House, Steny Hoyer, said Wednesday.

Trump demanded that the whistleblower who reported the president's alleged pressure on Ukraine be exposed, even though the identity of whistleblowers is protected by law.

Impeachment becomes campaign message

On Twitter, which Trump is using to bombard the public with conspiracy theories about a secret "deep state" trying to remove him from power, the president argued that the whistleblower had been shown to be partisan and inaccurate.

"The Whistleblower's facts have been so incorrect about my 'no pressure' conversation with the Ukrainian President, and now the conflict of interest and involvement with a Democrat Candidate, that he or she should be exposed and questioned," Trump tweeted.

In another tweet, Trump dismissed the impeachment process as a Democratic bid to influence the election, saying "their total focus is 2020, nothing more."

But Trump, who broke with precedent by campaigning for reelection almost from the moment he took office in 2017, is himself pouncing on the impeachment as the new cornerstone of his 2020 effort.

He and the Republican Party have pushed hard to raise funds off the back of their accusation of unfair treatment from the Democratic lower house in Congress.

And on Thursday and Friday, Trump will take that message to his core supporters when he holds campaign rallies in Minneapolis and in Louisiana.

Even if the House impeaches Trump, it remains unlikely that the Republican-led Senate would convict him in the subsequent trial.

However, Trump's already turbulent presidency would be forever associated with the impeachment.

Ukrainian phone call

House Speaker Nancy Pelosi formally launched the impeachment inquiry last month after revelations Trump pressured Ukraine's President Volodymyr Zelensky in a July 25 phone call.

In the call, Trump asked Zelensky to look into what the US leader said were corrupt business deals involving Biden.

Democrats say that Trump tried to coerce Zelensky by holding back US military aid to Ukraine. Trump says there was no quid pro quo and that his only desire is to combat corruption.

He subsequently said publicly he would also like China to investigate Biden, something critics say bolsters the allegation that Trump is seeking foreign help to discredit opponents.

On Tuesday, the Trump administration blocked a potentially major witness, ambassador to the European Union Gordon Sondland, from testifying before Congress. Democrats then slapped Sondland with a subpoena to appear on October 16.

"The failure to produce this witness, the failure to produce these documents" was "additional strong evidence of obstruction," House Intelligence Committee chairman Adam Schiff said.

Lawmakers want to hear on Friday from another key witness: former US ambassador to Kiev Marie Yovanovitch, who is scheduled to appear before the House Intelligence Committee.

US media has reported that Trump removed her from her post because she opposed his efforts to get Ukraine to investigate Biden.