Trump rejects impeachment charges as an affront to US Constitution

President Donald Trump on Monday rejected the Democratic-led House of Representatives' impeachment charges, describing the allegations that he had abused his power and obstructed Congress as affronts to the US Constitution that must be rejected.

"The Senate should speedily reject these deficient articles of impeachment and acquit the president," an executive summary of Trump's pre-trial brief said in the Republican president's first comprehensive defence before his Senate trial begins.

Trump, only the fourth of 45 American presidents to face the possibility of being ousted by impeachment, is charged with abusing the powers of his office by asking Ukraine to investigate a Democratic political rival, Joe Biden, and obstructing a congressional inquiry into his conduct.

The executive summary asserted that the House Democrats' "novel theory of 'abuse of power'" was not an impeachable offence and supplanted the Constitutional standard of "Treason, Bribery, or other high Crimes and Misdemeanors."

"House Democrats' newly-invented 'abuse of power' theory collapses at the threshold because it fails to allege any violation of law whatsoever," the summary said.

It rejected the obstruction of Congress charge as "frivolous and dangerous," saying the president exercised his legal rights by resisting congressional demands for information, also known as subpoenas.

"House Democrats propose removing the President from office because he asserted legal rights and privileges of the Executive Branch against defective subpoenas - based on advice from the Department of Justice," it said. "Accepting that theory would do lasting damage to the separation of powers" between the executive, legislative and judicial branches of government.

It also accused the House Democrats of conducting a rigged process, said they succeeded in proving only that Trump had done nothing wrong and argued, as the White House has repeatedly, that this was an effort to overturn Trump's 2016 election victory.

While the Republican-controlled Senate is highly unlikely to remove Trump from office, it is important for the Republican president to diminish the Democratic accusations as a partisan witch-hunt. He needs to limit the political damage to his re-election bid as he seeks a second term in November.

Trump's legal team says he was well within his constitutional authority to press Ukraine President Volodymyr Zelenskiy last year to investigate Biden and his son Hunter as part of what Trump says was an anti-corruption drive. The Bidens deny any wrongdoing and Trump's allegations have been widely debunked.

Democrats say Trump abused his power by withholding US military assistance to Ukraine as part of a pressure campaign and obstructed Congress by refusing to hand over documents and barring administration officials from testifying, even when subpoenaed by House investigators.

Trump's team says he is protected by the US Constitution's separation of powers provisions.

In a 111-page document filed before the Senate trial begins in earnest on Tuesday, Democratic lawmakers laid out their arguments against Trump, saying the president must be removed from office to protect national security and preserve the country's system of government.

Seeking to show he is still conducting presidential business despite the trial, Trump is scheduled to depart late on Monday for Davos, Switzerland, to join global leaders at the World Economic Forum. Some advisers had argued against him making the trip.