WASHINGTON (REUTERS) - United States House Speaker Nancy Pelosi on Wednesday (Dec 18) called Mr Donald Trump a threat to American democracy who left Congress no choice but to impeach him as bitterly divided lawmakers debated before historic votes on charges accusing the Republican President of abusing his power and obstructing Congress.

With billowing partisan differences on full display, the Democratic-led House of Representatives launched a planned six hours of debate on the two articles of impeachment arising from the President's actions towards Ukraine.

Mr Trump would become the third US president ever to be impeached.

That would set the stage for a trial in the Republican-led Senate next month, with House members acting as prosecutors and senators as jurors.

A conviction would result in his removal from office, but the Senate's top Republican has said there is "no chance" of that happening.

On the House floor, Ms Pelosi read the US Pledge of Allegiance, then said: "Today we are here to defend democracy for the people," to applause from fellow Democratic lawmakers.

As the debate unfolded, Mr Trump on Twitter called the proceedings "an assault on America" and on his party.

Mr Trump, who has denied wrongdoing, later added of Ms Pelosi: "Will go down in history as worst Speaker."

While the House twice previously voted to impeach presidents - Mr Bill Clinton in 1998 and Mr Andrew Johnson in 1868 - no president has ever been removed from office via impeachment.

"It is a matter of fact that the President is an ongoing threat to our national security and the integrity of our elections - the basis of our democracy," Ms Pelosi said.

House Democrats accuse Mr Trump, 73, of abusing his power by asking Ukraine to investigate former vice-president Joe Biden, a leading contender for the Democratic nomination to face Mr Trump in the November 2020 election.

He is also accused of obstructing the House investigation by directing administration officials and agencies not to comply with subpoenas for testimony and documents related to impeachment.

Republican Representative Mike Rogers said: "The matter before the House today is based solely on a fundamental hatred of our President. It's a sham, a witch hunt - and it's tantamount to a coup against the duly elected President of the United States."

The debate underscored the deep divide in Congress over Trump's conduct during his tumultuous presidency and the larger political schism dividing the US.

Separate votes on the two charges are expected in the early evening. The votes are expected to fall almost entirely along party lines, with Democrats in favour and Republicans opposed.

In speeches on the House floor, Republicans defended the President and accused Democrats of seeking to use an unfair, rigged process to nullify the results of the 2016 election in which Mr Trump defeated Democrat Hillary Clinton. The House voted down a Republican bid to adjourn a motion accusing Democrats of violating the chamber's rules.

"If we do not act now, we would be derelict in our duty. It is tragic that the President's reckless actions make impeachment necessary," Ms Pelosi added.

"He gave us no choice. What we are discussing today is the established fact that the president violated the Constitution.

Republican Representative Barry Loudermilk invoked the name of Jesus during the debate.

Referring to the ancient Roman provincial governor who presided over the trial of Jesus and ordered the crucifixion, Mr Loudermilk said, "During that sham trial, Pontius Pilate afforded more rights to Jesus than the Democrats have afforded to this President in this process."

Republican Representative Mike Kelly compared the impeachment to the Japanese attack on the US naval base at Hawaii's Pearl Harbor in 1941, calling the House proceedings another "date that will live in infamy", similar to the words then Democratic President Franklin Roosevelt used to describe the raid that led to America's entry into World War II.

Impeachment is an extraordinary check on presidential power spelled out in the Constitution enabling Congress to remove presidents who commit "high crimes and misdemeanours".

"The founders' great fear of a rogue or corrupt president is the very reason why they enshrined impeachment in the Constitution," Ms Pelosi said, calling the gathered lawmakers the"custodians of the Constitution".

The House vote to approve the rules to formally begin debate was 228-197, a possible preview of the later votes on the articles of impeachment, with only two Democrats - Representatives Collin Peterson and Jeff Van Drew - voting no and no Republicans voting in favour.

Mr Van Drew has announced plans to become a Republican. Representative Justin Amash, an independent who left the Republican Party in July, joined the Democrats in the vote.

Vice-President Mike Pence, speaking in Michigan, touted the economy under Mr Trump and called the House proceedings "a disgrace".

"They are trying to impeach this President because they know they can't defeat this President (in 2020)," he said.

But House Judiciary Committee chairman Jerrold Nadler said on the House floor: "We cannot rely on the next election as a remedy for presidential misconduct when the President threatens the very integrity of that election. He has shown he will continue to put his selfish interests above the good of the country. We must act without delay."

WHISTLE-BLOWER'S COMPLAINT

Fearful of political blowback, senior Democrats long resisted an impeachment inquiry, even after Special Counsel Robert Mueller outlined episodes of Mr Trump seeking to impede the federal investigation that documented Russian interference in the 2016 election to boost his candidacy.

But after a whistle blower from the US intelligence community brought to light a July 25 phone call in which Mr Trump asked Ukrainian President Volodymyr Zelensky to investigate Mr Biden, House Democrats launched an impeachment inquiry in September and moved swiftly to take testimony from current and former administration officials.

Democrats said Mr Trump held back US$391 million (S$530 million) in security aid intended to combat Russia-backed separatists and a coveted White House meeting for Mr Zelensky as leverage to coerce Kiev into interfering in the 2020 election by smearing Mr Biden.

On the July telephone call, Mr Trump asked Mr Zelensky to investigate Mr Biden and his son Hunter Biden as well as a discredited theory promoted by the President and beneficial to Russia that Democrats conspired with Ukraine to meddle in the 2016 election. Mr Hunter Biden had joined the board of Ukrainian energy company Burisma while his father was vice-president.

Mr Trump has accused the Bidens of corruption, without offering evidence. They have denied wrongdoing.

Senator Lindsey Graham, the Senate Judiciary Committee's Republican chairman, said on Wednesday that he would want any impeachment trial to be quick and with no new witnesses.

Removing Mr Trump from office would require a two-thirds majority of those present and voting in the 100-member Senate, meaning at least 20 Republicans would have to vote to convict the President.