Donald Trump must be removed from office to safeguard the 2020 election, preserve the constitution and protect national security, according to an impeachment trial brief filed by House Democrats.

The US president abused the powers of his office, “abandoned his oath to faithfully execute the laws and betrayed his public trust” in his dealings with Ukraine, the memorandum stated.

It also argued that Mr Trump’s behaviour in seeking to pressure Ukraine to launch an investigation into his political rival was “the worst nightmare” of the framers of the US constitution.

“The Senate should convict and remove President Trump to avoid serious and long term damage to our democratic values and the nation’s security,” the document added.

Mr Trump’s legal team immediately hit back by arguing that impeachment was “a dangerous attack on the right of the American people to freely choose their president”.

“This is a brazen and unlawful attempt to overturn the results of the 2016 election and interfere with the 2020 election, now just months away,” the president’s lawyers said.

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The 111-page impeachment trial brief, filed ahead of the 5pm deadline on Saturday, was signed by House intelligence committee chairman Adam Schiff and six other House managers.

It set out their arguments supporting charges of abuse of power and obstruction of Congress against the president.

“President Donald Trump used his official powers to pressure a foreign government to interfere in a United States election for his personal political gain, and then attempted to cover up his scheme by obstructing Congress’s investigation into his misconduct,” it states.

“The Constitution provides a remedy when the President commits such serious abuses of his office: impeachment and removal.

“The Senate must use that remedy now to safeguard the 2020 U.S. election, protect our constitutional form of government, and eliminate the threat that the President poses to America’s national security.”

House Democrats also described Mr Trump in the memorandum as “an aberration among presidents” because of his refusal to cooperate with the impeachment investigation.

They added: “The case against the president of the United States is simple, the facts are indisputable, and the evidence is overwhelming.”

In comments reported by White House pool journalists, senior Trump administration officials who asked to remain anonymous described the articles of impeachment as “defective in their entirety” and the “product of invalid proceedings”.

They claimed that the abuse of power charge if upheld “would do lasting damage to the separation of powers”. The Trump response to the impeachment summons will “address both the 21 April and 25 July phone calls”. they said.

Regarding the charge that Mr Trump obstructed congress by ordering administration officials not to provide evidence to impeachment investigators, an official told pool reporters: “The president’s assertion of legitimate executive branch confidentiality interests, grounded in the separation of powers, cannot constitute obstruction.”

The official went on to claim that Mr Trump had “acted with extraordinary and unprecedented transparency by declassifying and releasing the transcript of the 25 July call that is at the heart of this matter” – but the document released by the president was not actually a transcript, despite his frequent protestations that it was one.

The filing came after documents revealed an associate of Mr Trump’s personal lawyer had sent messages to the president’s donors about his activities in Ukraine and also about Joe Biden, the Democratic frontrunner for that party’s 2020 nomination.

Lev Parnas tipped off a high-rolling donor that a “bomb [was] dropping” as Marie Yovanovitch, the US ambassador, was forced out of her role.T

The Soviet-born businessman also appeared to have received financial aid from Trump backers to fund his work – which he said this week was “all about 2020” and ensuring Mr Trump is re-elected.