WASHINGTON (NYTIMES) - United States President Donald Trump's legal defence team forcefully denied on Saturday (Jan 18) that he abused his power by pressuring a foreign government to investigate his political rivals, calling the two impeachment charges against him a "brazen and unlawful" attempt to hurt his chances of re-election.

The defiant rejection of the accusations came in response to an official summons issued in the past week by the Senate, notifying Mr Trump that he faces removal from office if he is convicted.

In a six-page letter, Mr Trump's first formal response to the charges against him, his lawyers denounced the impeachment case brought by House Democrats as constitutionally and legally invalid, and driven by malice toward him.

"The articles of impeachment submitted by House Democrats are a dangerous attack on the right of the American people to freely choose their president," the document says.

"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 did not deny any of the core facts underlying Democrats' charges, conceding what ample evidence has shown, that he withheld US$391 million (S$526 million) in aid from Ukraine and asked the country's president to investigate former vice-president Joe Biden and his son, Hunter.

But they said Mr Trump broke no laws and was acting entirely appropriately and within his powers when he did so, echoing the President's repeated protestations of his own innocence.

They argued that Mr Trump was not seeking political advantage but working to root out corruption in Ukraine.

"President Trump categorically and unequivocally denies each and every allegation in both articles of impeachment," Mr Pat Cipollone, White House counsel, and Mr Jay Sekulow, Mr Trump's personal lawyer, wrote.

Mr Trump's response came shortly after the House impeachment managers formally outlined their case for Mr Trump's removal from office, arguing in a lengthy legal filing that the Senate should convict him for abuse of power and obstruction of Congress.

In the 46-page trial memorandum, the House impeachment managers asserted that beginning in the spring, Mr Trump undertook a corrupt campaign to push Ukraine to publicly announce investigations of his political rivals, withholding as leverage nearly US$400 million in military aid and a White House meeting.

He then sought to conceal those actions from Congress, they said, refusing to cooperate with a House impeachment inquiry and ordering administration officials not to testify or turn over documents requested by investigators.

"President Trump's conduct is the framers' worst nightmare," the managers wrote, framing their argument in constitutional terms.

The legal back-and-forth on Saturday offered a preview of the strategies both sides will employ starting in the coming week, when the Senate opens oral arguments in only the third impeachment trial of a president in the nation's history.

Addressing head-on the political dynamics of the Senate, where majority Republicans have denounced the impeachment inquiry, House managers warned that voters and future generations would sit in judgment of their actions.

"History will judge each senator's willingness to rise above partisan differences, view the facts honestly, and defend the Constitution," they wrote. "The outcome of these proceedings will determine whether generations to come will enjoy a safe and secure democracy in which the president is not a king."

The filing from the House Democrats repeated many of the same arguments they laid out last fall in a report on the findings of their impeachment inquiry. But the managers' brief provided a glimpse of their strategy for the high-stakes legal and political fight ahead.

The heavily footnoted document, formatted in the style of a courtroom filing, was headlined "In re Impeachment of President Donald J. Trump" and addressed to the Senate, "sitting as a court of impeachment".

The memorandum laid out the evidence and legal arguments the managers intend to present in oral arguments on the floor of the Senate, likely beginning Wednesday.

The filing also included an additional 60 pages of facts the managers deemed material to their case. As they have said for weeks, the president's lawyers asserted in Saturday's short filing that the articles of impeachment against Mr Trump are "invalid on their face" because they do not accuse the president of breaking the law.

In Saturday's document, the President's legal team also rejected the charge that Mr Trump is guilty of obstruction of Congress.

They argued that Mr Trump's attempts to prevent witnesses from testifying in what the President has called a "sham" impeachment inquiry is a legitimate exercise of executive privilege that is essential to guard the authority and prerogatives of the presidency.

And they once again attacked the process by which House Democrats impeached Mr Trump, accusing them of denying the President his due process rights.

The President's legal team faces a deadline of noon on Monday to produce a more comprehensive legal brief laying out the defence case they will make on the floor of the Senate.

Mr Cipollone and Mr Sekulow will lead the President's defence at trial.

The White House announced on Friday that the team will also include Mr Ken Starr, the former independent counsel whose investigation of President Bill Clinton led to his impeachment; Mr Robert Ray, who succeeded Mr Starr; and Mr Alan Dershowitz, a celebrity defence lawyer.