Democratic White House hopeful Joe Biden has called for US President Donald Trump's impeachment, saying he has "betrayed" the country.

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

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

However, President Trump showed no sign of buckling under pressure from the Democratic Party inquiry into his alleged bid to damage Mr Biden by strong-arming Ukraine to investigate the former vice president.

Having threatened a constitutional crisis by refusing to cooperate with the congressional investigation, Mr Trump predicted that the row would end up "being a big Supreme Court case".

He told reporters in the White House that his Republican Party was being "treated very badly".

Democrats accuse Mr 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.

On Twitter, which Mr Trump is using to bombard the public with conspiracy theories about a "deep state" aiming to eject him, the president argued that the whistleblower behind the impeachment case 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," Mr Trump tweeted.

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

But Mr 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.

Over the next two days, President Trump will take that message to his core supporters when he holds campaign rallies in Minneapolis and in Louisiana.

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

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

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

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

Democrats say that Mr Trump tried to coerce Mr Zelensky by holding back US military aid to Ukraine.

Mr 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 Mr Biden, something critics say bolsters the allegation that Mr 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 responded by issuing a subpoena to Mr Sondland to appear on 16 October.