The leader of congressional opposition to Donald Trump has declared that his attempts to stall an impeachment inquiry will prove futile, warning the US president: "You are not above the law."

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Nancy Pelosi, the speaker of the House of Representatives, said Democrats intend to move full speed ahead with their investigation despite the White House declaring it illegitimate, pushing the US towards a constitutional crisis.

White House counsel Pat Cipollone wrote to Pelosi and other Democratic leaders on Tuesday: "Given that your inquiry lacks any legitimate constitutional foundation, any pretense of fairness, or even the most elementary due process protections, the executive branch [White House] cannot be expected to participate in it."

Cipollone's eight-page missive objected that the House did not formally vote to begin the impeachment inquiry, breaking with precedent set in the inquiries into Richard Nixon and Bill Clinton. It also accused investigators of denying Trump due process, including the right to see evidence and cross-examine witnesses, and threatened to end White House cooperation with Congress on important oversight matters.

The letter set the stage for a historic clash between the executive and legislative branches of the US government. Pelosi, who announced the impeachment inquiry last month after it emerged that Trump pressed Ukraine's leader to investigate political rival Joe Biden, delivered a scathing response.

"The White House letter is only the latest attempt to cover up his betrayal of our democracy, and to insist that the president is above the law," she said. "This letter is manifestly wrong, and is simply another unlawful attempt to hide the facts of the Trump administration's brazen efforts to pressure foreign powers to intervene in the 2020 elections."

The speaker added: "The White House should be warned that continued efforts to hide the truth of the president's abuse of power from the American people will be regarded as further evidence of obstruction. Mr President, you are not above the law. You will be held accountable."

Article 1 of the United States constitution gives the House of Representatives the sole power to initiate impeachment and the Senate the sole power to try impeachments of the president. A president can be impeached if they are judged to have committed "treason, bribery, or other high crimes and misdemeanors" - although the constitution does not specify what "high crimes and misdemeanors" are.

The process starts with the House of Representatives passing articles of impeachment. A simple majority of members need to vote in favour of impeachment for it to pass to the next stage. Democrats currently control the house, with 235 representatives.

The chief justice of the US supreme court then presides over the proceedings in the Senate, where the president is tried, with senators acting as the jury. For the president to be found guilty two-thirds of senators must vote to convict. Republicans currently control the Senate, with 53 of the 100 senators.

Two presidents have previously been impeached, Bill Clinton in 1998, and Andrew Johnson in 1868, though neither was removed from office as a result. Richard Nixon resigned in 1974 before there was a formal vote to impeach him.

Martin Belam

Trump aides have begun honing their approach after two weeks of what some allies have described as a listless and unfocused response to the inquiry. The letter appeared to put the emphasis on political rebuttal rather than structured legal argument - signalling a new strategy to counter the impeachment threat by openly defying Congress's right to investigate the president for high crimes and misdemeanours.

The House Republican leader, Kevin McCarthy, said: "President Trump is right to call out this rushed process because Democrats refuse to protect the transparency and basic fairness that have been integral to previous impeachment proceedings."

But the letter was widely scorned by political analysts and legal experts. George Conway, a lawyer married to Trump adviser Kellyanne Conway, tweeted: "I cannot fathom how any self-respecting member of the bar could affix his name to this letter. It's pure hackery, and it disgraces the profession."

Gregg Nunziata, the former chief nominations counsel for Republicans on the Senate judiciary committee, added on Twitter: " Wow. This letter is bananas. A barely-lawyered temper tantrum. A middle finger to Congress and its oversight responsibilities. No member of Congress should accept it. Things are bad. Things will get worse."

Cipollone's intervention came after the Trump administration abruptly blocked a key witness in the Ukraine scandal from appearing before the congressional impeachment inquiry.

The state department said the US ambassador to the European Union, Gordon Sondland, a Trump political donor, would not be allowed to appear, even though he had already travelled from Europe to testify behind closed doors in Washington. Trump decried the Democratic-led inquiry into whether he abused his office in the pursuit of personal political gain as a "kangaroo court".

Democrats condemned the move, calling it an attempt to obstruct their inquiry, and issued a subpoena for Sondland, seeking documents by 14 October and a deposition on Capitol Hill on 16 October.

Schiff said the ambassador's no-show was "yet additional strong evidence" of obstruction of Congress by Trump and the secretary of state, Mike Pompeo, that will only strengthen a possible impeachment case.

Meanwhile, Trump's personal lawyer, Rudy Giuliani, has said he will not testify before the House intelligence committee unless committee members vote to remove its chairman, Adam Schiff. "I wouldn't testify in front of that committee until there is a vote of Congress and he is removed," Giuliani told the Washington Post. "Let them hold me in contempt. We'll go to court. We'll challenge the contempt."

Related: Refuse, block, stonewall - but Trump's strategy leaves little margin for error

He added: "The position I'm stating is now the position of the administration."

The House committees leading the fast-moving investigation intend to call and subpoena a number of Trump administration witnesses. The investigation could lead to the approval of articles of impeachment against Trump in the House. A trial on whether to remove him from office would then be held in the Senate, where Republicans have the upper hand.

Opinion polls show some worrying signs for Trump. According to the Politico/Morning Consult survey, 50% of voters would support Trump's removal, compared to 43% who would not. A Washington Post/Schar School poll found 49% of Americans think Trump should be removed from office.

Yet on Wednesday, in his latest barrage of tweets, Trump cited a mysterious statistic to fit his worldview: "Only 25 percent want the President Impeached, which is pretty low considering the volume of Fake News coverage, but pretty high considering the fact that I did NOTHING wrong. It is all just a continuation of the greatest Scam and Witch Hunt in the history of our Country!"

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