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.”

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.”

Q&A How do you impeach the US president? Show 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 US Constitution does not specify what “high crimes and misdemeanors” are.

The formal process starts with the House of Representatives passing articles of impeachment, the equivalent of congressional charges. According to arcane Senate rules, after the House notifies the Senate that impeachment managers have been selected, the secretary of the Senate, Julie Adams, tells the House that the Senate is ready to receive the articles. Then impeachment managers appear before the Senate to “exhibit” the articles, and the Senate confirms it will consider the case. The presiding officer of the Senate notifies the supreme court chief justice, John Roberts, of the impending trial. Roberts arrives in the Senate to administer an oath to members. The presiding officer will then administer this oath to senators: “I solemnly swear that in all things appertaining to the trial of the impeachment of Donald Trump, now pending, I will do impartial justice according to the constitution and laws, so help me God.” The Senate must vote on a resolution laying out ground rules for the trial including who the key players will be, how long they will get to present their cases and other matters. After the Senate is “organized”, the rules decree, “a writ of summons shall issue to the person impeached, reciting said articles, and notifying him to appear before the Senate upon a day and at a place to be fixed by the Senate”. A president has never appeared at his own impeachment trial. Trump will be represented by the White House counsel, Pat Cipollone, and his personal lawyer Jay Sekulow, among others. After the oath, the trial proper will begin. Senators may not speak during the proceedings but may submit written questions. The question of witnesses and other matters would be decided on the fly by majority vote. A time limit for the proceedings will be established in the initial Senate vote. The senators will then deliberate on the case. In the past this has happened behind closed doors and out of public view. The senators vote separately on the two articles of impeachment – the first charging Trump with abuse of power, the second charging him with obstruction of Congress. A two-thirds majority of present senators – 67 ayes if everyone votes – on either article would be enough to convict Trump and remove him from office. But that would require about 20 Republicans defections and is unlikely. The more likely outcome is a Trump acquittal, at which point the process is concluded. 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. Tom McCarthy in New York

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.”

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!”﻿