A defiant Donald Trump rejected blame Tuesday for a deadly assault on Congress by his supporters, but cracks emerged in the president's Republican support with several now backing his removal on the eve of an all-but-certain historic second impeachment.

Vice President Mike Pence gave the besieged Trump a lifeline by saying he would not invoke the 25th Amendment that allows him and the Cabinet to strip a sitting president of his powers.

"I do not believe that such a course of action is in the best interest of our nation," Pence wrote in a letter to House Speaker Nancy Pelosi.

US President Donald Trump leaves for a trip to Texas, breaking his isolation in the White House US President Donald Trump leaves for a trip to Texas, breaking his isolation in the White House Photo: AFP / Brendan Smialowski

The House of Representatives delivered a stinging rebuke to Trump anyway, voting almost entirely along party lines to call on Pence to take action and remove the president.

But with the 25th Amendment avenue dead in the water, an impeachment of Trump on Wednesday appeared virtually assured.

Pelosi quickly announced her managers for the process, and a House impeachment vote has been scheduled for roughly 3:00 pm (2000 GMT).

The single charge of "incitement of insurrection" -- over Trump's January 6 speech in which he claimed he was the real winner of the November election, then urged supporters to march on Congress and "fight" -- is all but sure to get majority support.

US National Guard soldiers guard the grounds of the US Capitol days after a Trump supporters stormed the building US National Guard soldiers guard the grounds of the US Capitol days after a Trump supporters stormed the building Photo: AFP / Daniel SLIM

Earlier Trump traveled to Alamo, Texas, and although he urged "peace and calm" during a visit to the US-Mexico border wall, his overall message was of refusal to take blame for last week's violence.

On January 6 the pro-Trump crowd attacked the Capitol, fighting with police, ransacking offices, and briefly forcing terrified lawmakers -- and Pence -- to abandon a session certifying Democrat Joe Biden's election victory.

An unapologetic Trump insisted that "everybody" thought his speech was "totally appropriate."

Trump dubbed his likely impeachment a "continuation of the greatest witch hunt in the history of politics."

The US Capitol is open to lawmakers and staff, but under tight police and military security The US Capitol is open to lawmakers and staff, but under tight police and military security Photo: AFPTV / Bastien INZAURRALDE

And he warned that while "you have to always avoid violence," his supporters are furious.

"I've never seen such anger," he said.

Democrats are all but sure to approve impeachment in the House.

US President Donald Trump says impeachment is causing 'tremendous anger' US President Donald Trump says impeachment is causing 'tremendous anger' Photo: AFP / Brendan Smialowski

It seemed unlikely though that the Republican-controlled Senate would be called into an emergency session to put Trump on trial before his term runs out on January 20.

However, several in the party that Trump has held in thrall for the last four years have broken ranks.

According to The New York Times, the powerful Senate majority leader, Senator Mitch McConnell, has said privately he believes Trump did commit impeachable offenses.

In the House, the number three Republican Liz Cheney said she would be voting to impeach, and called Trump's actions "a betrayal" of his office.

US Vice President Mike Pence said he was opposed to invoking the 25th Amendment, a process that could have led to the ouster of President Donald Trump before his term ends on January 20, 2021 US Vice President Mike Pence said he was opposed to invoking the 25th Amendment, a process that could have led to the ouster of President Donald Trump before his term ends on January 20, 2021 Photo: AFP / MANDEL NGAN

This came after top House Republican Kevin McCarthy said members would not be required to toe the party line on the vote -- a significant weakening of support for Trump.

Four other House Republicans have now also publicly stated they will vote for impeachment.

Barred from Twitter and Facebook -- two platforms integral to his shock rise to power in 2016 -- Trump is for the first time struggling to shape the news message, a censoring by Big Tech that he called a "catastrophic mistake."

His social media woes continued late Tuesday when video-sharing giant YouTube said it was suspending his official account for at least a week, out of concern his videos could incite violence.

Ever since the November 3 election, the Republican real estate tycoon has been obsessively pushing his lie that Biden stole the election.

But his speech to supporters last week and the crowd's attack on Congress, which included fatally wounding a policeman, proved beyond the pale even for some of his staunchest supporters.

Major representatives of the corporate and sporting world have rejected Trump, while the Republican Party is splitting between ultra-loyalists and a growing number of lawmakers who see him as a liability.

Trump has yet to congratulate Biden or urge his supporters to stand behind the incoming president after he is inaugurated -- a gesture of political unity considered routine after US elections.

With a string of cabinet officials quitting the government -- most recently the acting head of the Department of Homeland Security Chad Wolf on Monday -- it's also clear that Trump's grip on power is tenuous.

The Republican-controlled Senate, however, is in recess until January 19 and its leadership says there is no way to rush through an impeachment trial before Biden takes over the following day.

This would mean that Trump, who was already acquitted in the Senate last year after his first impeachment trial, would not be forced out of office early.

Not even all Democrats are gunning whole heartedly for a trial, worried that this would overshadow Biden's first days in office.

The new president will already face the challenges of an out-of-control Covid-19 pandemic, the stumbling vaccination program, a shaky economy, and now the aftermath of violent political opposition from parts of Trump's huge voter base.

Meanwhile official Washington has begun to take on the appearance of a fortress, with security substantially boosted on Capitol Hill one week before Biden is sworn in.