US President Donald Trump's legal team issued a fiery response yesterday before opening arguments in his impeachment trial while House Democrats laid out their case in forceful fashion, saying the President had betrayed public trust with behaviour that was the "worst nightmare" of the founding fathers.

The duelling statements previewed arguments both sides intend to make once Trump's impeachment trial begins in earnest on Wednesday. Their challenge will be to make a case that appeals to senators who will render the verdict and for an American public bracing for a presidential election in 10 months.

The House's 111-page brief pulled together the private and public testimony of a dozen witnesses — ambassadors and national security officials at high levels of government — who raised concerns about the President's actions with Ukraine. The document underscored the extent to which the impeachment proceedings are a political rather than conventional legal process.

The Trump team similarly offered a taste of the rhetoric expected to be deployed by the President's defenders in the Senate.

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In their brief, the House managers overseeing the prosecution wrote it was clear the "evidence overwhelmingly establishes" Trump is guilty of both charges for which he was impeached last month: abuse of power and obstruction of Congress. "The only remaining question is whether the members of the Senate will accept and carry out the responsibility placed on them by the Framers of our Constitution and their constitutional Oaths," the brief states.

The Trump team called the Senate's formal impeachment summons to the two articles of impeachment "a dangerous attack on the right of the American people to freely choose their president".

Trump's legal team, led by White House counsel Pat Cipollone and Trump personal lawyer Jay Sekulow, is challenging the impeachment on procedural and constitutional grounds, claiming Trump has been mistreated by House Democrats and that he did nothing wrong.

"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 filing states. The lawyers said Trump "categorically and unequivocally" denies the allegations and encouraged lawmakers to reject "poisonous partisanship" and "vindicate the will of the American people" by rejecting both articles.

Trump's answer to the summons was the first salvo in what will be several rounds of arguments before the trial formally begins.

At issue in the impeachment cases are allegations Trump requested his Ukraine counterpart to investigate political rival Joe Biden at the same time his administration withheld hundreds of millions of dollars in aid from the former Soviet republic.

In other developments

•New documents released by House Democrats suggest Devin Nunes, the top Republican on the House Intelligence Committee, was more deeply involved than was previously known in efforts by allies of President Trump to dig up dirt in Ukraine on former Vice President Joe Biden. The materials were provided to the House by Lev Parnas, a Florida businessman who worked with Trump's personal lawyer Rudy Giuliani to try to persuade the Ukrainian government to launch an investigation into Biden. Parnas and his business partner, Igor Fruman, were indicted last year on charges of conspiracy, making false statements and falsification of records. The men have pleaded not guilty.

•Four Republican senators indicated they may join Democrats to call new witnesses to give evidence against President Trump. John Bolton, Trump's former national security adviser, and Mick Mulvaney, the acting White House chief of staff, top the list of potential witnesses Democrats want to hear from over Trump's withholding of US$400 million (NZ$605m) in military aid to Ukraine while seeking an investigation into political rival Joe Biden. Witnesses will only be called if at least 51 of the 100 senators vote for a motion to hear from them. There are 47 Democrats and independents, meaning the support of four Republicans would be needed.

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- AP & Telegraph Group Ltd