US President Donald Trump's legal team has issued a fiery response ahead of opening arguments in his impeachment trial.
It comes as House Democrats lay out their case in forceful fashion, saying the president betrayed public trust with behaviour that was the "worst nightmare" of the founding fathers.
The duelling filings previewed arguments both sides intend to make once Mr Trump's impeachment trial begins in earnest on Tuesday in the Senate. Their challenge will be to make a case that appeals to the 100 senators who will render the verdict and for an American public bracing for a presidential election in 10 months.
"President Donald J. Trump used his official powers to pressure a foreign government to interfere in a United States election for his personal political gain," the House prosecutors wrote, "and then attempted to cover up his scheme by obstructing Congress's investigation into his misconduct."
Mr Trump's legal team, responding to the Senate's official summons for the trial, said the president "categorically and unequivocally" denies the charges of abuse and obstruction against him.
"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 filing states.
Stripped of legalese and structured in plain English, the documents underscored the extent to which the impeachment proceedings are a political rather than conventional legal process.
They are the first of several filings expected in coming days as senators prepare to take their seats for the rare impeachment court.
Senators swore an oath to do "impartial justice"' as the chamber convenes to consider the two articles of impeachment approved by the House last month as Mr Trump's presidency and legacy hangs in balance.
One Republican whose votes are closely watched, Sen. Lisa Murkowski of Alaska, acknowledged Saturday the political pressure bearing on them.
"I'm going to take my constitutional obligations very, very seriously," she told reporters from Anchorage on a call.
The House's 111-page brief outlined the prosecutors' narrative, starting from Mr Trump's phone call with Ukraine and relying on 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.
The House managers wrote: "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 Trump team called the two articles of impeachment "a dangerous attack on the right of the American people to freely choose their president."
Mr Trump's team encouraged lawmakers to reject "poisonous partisanship" and "vindicate the will of the American people" by rejecting both articles of impeachment approved by the House.
The Senate is still debating the ground rules of the trial, particularly the question of whether there will be new witnesses as fresh evidence emerges over Mr Trump's Ukraine actions that led to impeachment.
New information from Lev Parnas, an indicted associate of Mr Trump lawyer Rudy Giuliani, is being incorporated in the House case. At the same time, Senate Democrats want to call John Bolton, the former national security adviser, among other potential eyewitnesses, after the White House blocked officials from appearing in the House.
With Republicans controlling the Senate 53-47, they can set the trial rules - or any four Republicans could join with Democrats to change course. Sen Murkowski told reporters she wants to hear both sides of the case before deciding whether to call for new witnesses and testimony.
"I don't know what more we need until I've been given the base case," Sen Murkowski said.
The House's impeachment managers are working through the weekend and will be at the Capitol midday Sunday (local time) to prep the case.
Mr Trump's answer to the summons was the first salvo in what will be several rounds of opening arguments.
Mr Trump will file a more detailed legal brief on Monday, and the House will be able to respond to the Trump filing on Tuesday.
Mr Trump's team led by White House counsel Pat Cipollone and Mr Trump personal lawyer Jay Sekulow, is challenging the impeachment on both procedural and constitutional grounds, claiming Mr Trump has been mistreated by House Democrats and that he did nothing wrong.
The filings came a day after Mr Trump finalised his legal team, adding Ken Starr, the former independent counsel whose investigation into President Bill Clinton led to his impeachment, and Alan Dershowitz, a Harvard law professor emeritus who intends to make constitutional arguments.
White House attorneys and Mr Trump's outside legal team have been debating just how political Monday's legal brief laying out the contours of Mr Trump's defense should be.
Some in the administration have echoed warnings from Senate Majority Leader Mitch McConnell, R-Ky., that the pleadings must be sensitive to the Senate's more staid traditions and leave some of the sharper rhetoric exhibited during the House proceedings to Twitter and cable news.
One Democratic aide said Saturday that Mr Trump's initial filing read more like a Trump campaign fundraising email than a legal document.
People close to the Trump legal team said Cipollone would deliver the president's opening argument before the Senate and that Sekulow would follow. Starr and Dershowitz would have "discrete functions" on the legal team, according to those close to the legal team, who were not authorized to discuss the strategy by name and spoke on condition of anonymity.
At issue in the impeachment case are allegations that Mr Trump asked Ukraine to announce an investigation of Democratic political rival Joe Biden at the same time the White House withheld hundreds of nearly $400 million in aid from the former Soviet republic as it faces a hostile Russia at its border. The Government Accountability Office said last week the administration violated federal law by withholding the funds to Ukraine. The money was later released after Congress complained.
The House brief said, "President Trump's misconduct presents a danger to our democratic processes, our national security, and our commitment to the rule of law. He must be removed from office.
Mr Trump's attorneys argue that the articles of impeachment are unconstitutional in and of themselves and invalid because they don't allege a crime. Under the Constitution impeachment is a political, not a criminal process, and the president can be removed from office if found guilty of whatever lawmakers consider "high crimes and misdemeanours."
TRUMP TRIAL COULD BE OVER IN TWO WEEKS
Bill Clinton's 1999 impeachment trial lasted five weeks; Andrew Johnson's went on for three months in 1868.
If the White House and Republicans have their way, President Donald Trump's trial will be over in two weeks, just in time for him to celebrate his expected acquittal in the February 4 State of the Union Address.
But that depends on Republicans being able to block Democrat demands to subpoena documents and witnesses that could strengthen the case against the president.
So far, Republicans, led by Mr Trump's tough protector, Senate Majority Leader Mitch McConnell, have the upper hand.
Mr Trump's trial for abuse of power and obstruction of Congress opened with a solemn, ceremonial reading of the charges in the Senate on Thursday, but the rules and schedule have not been set.
That will be decided on Tuesday, with the 100 senators debating voting on procedures: the time given to opening arguments from the prosecution and defense, and questioning by the senators - the jury in the case.
THE KEY ISSUE: WITNESSES
Democrats are demanding that the Senate agree to subpoena crucial documents and four current and former senior White House officials to testify.
They include former national security advisor John Bolton and Mr Trump acting chief of staff Mick Mulvaney.
Both are believed to have first-hand knowledge of what Mr Trump is accused of: using his powers illicitly to pressure Ukraine to help his 2020 reelection campaign by investigating his potential Democratic opponent Joe Biden.
The White House blocked the impeachment investigation in the House of Representatives from accessing the witnesses and documents, and shows no sign of giving in now.
McConnell says the issue won't be decided until after the trial's initial arguments and questioning take place, and has made clear he doesn't see the need anyway.
The White House signalled this week what it expects will happen: no witnesses. "I think it's extraordinarily unlikely it will be going beyond two weeks," a White House official told reporters.
He said there is no need to go any longer.
"The president should be acquitted. We think it's going to happen and going to happen readily."
CONTROLLING THE RULES
McConnell oversees a 53-47 Republican majority in the body, giving him all the power he needs to set the rules his way and deny the witnesses.
Unless four Republican senators break ranks, at the end of the arguments McConnell can defeat any motion to subpoena witnesses, and then can easily hold a vote to acquit Mr Trump - meeting the White House timeframe.
Democrats are pressing Republicans to support a witness resolution. "We have asked for four fact witnesses and three specific sets of relevant documents," Chuck Schumer, the senior Democrat in the Senate, said Thursday.
"So in the coming days, each of us, every one of us, Democrat and Republican, will face a choice about whether to begin this trial in the search of truth or in service of the president's desire to cover it up."
Democrats see several Republican senators as possibly willing to stand with them, but none have committed.
One, Maine Senator Susan Collins, came under heavy pressure after a video was widely circulated in the media and social networks showing her in 1999 saying about Clinton's trial: "I need more evidence. I need witnesses and further evidence to guide me to the right destination, to get to the truth." In a statement Thursday, Sen Collins said she hadn't decided yet. "I tend to believe having additional information would be helpful. It is likely that I would support a motion to call witnesses at that point in the trial just as I did in 1999." Another possible Republican crossover, Senator Mitt Romney, told journalists: "Barring some sort of surprise I'll vote in favour of hearing witnesses."
No other Republican has gone that far.
If four Republicans do cross over, that could add weeks to the trial - three in the Clinton case.
But Sen McConnell has another card up his sleeve: demanding witnesses that the White House wants, even if they have little bearing on Mr Trump's guilt or innocence.
Republicans have said they would call Mr Biden's son Hunter, whose association with a Ukraine energy firm was part of what Mr Trump wanted Kiev to investigate.
They also want to call the whistleblower whose August complaint about Mr Trump's Ukraine dealings set off the investigation that led to impeachment.
That would provoke a showdown.
Republican Senator Rand Paul told Politico that if Democrats get their witnesses, "my insistence will be not just one witness, but that the president should be able to call any witnesses that he deems necessary to his defense," Mr Paul said.
He meanwhile warned Republicans who don't support the White House witness demands.
"If you vote against Hunter Biden, you're voting to lose your election, basically. Seriously. That's what it is," he said.