Vice President Pence on Wednesday told Congress he will not block the certification of the Electoral College vote, likely removing the last obstacle to Joe Biden being certified the winner of the November presidential election.

"It is my considered judgment that my oath to support and defend the Constitution constrains me from claiming unilateral authority to determine which electoral votes should be counted and which should not," Pence wrote.

McConnell, speaking from the Senate floor, said that the allegations of fraud didn’t reach the standard for challenging the election results and warned of dramatic consequences if the effort were successful.

“If this election were overturned by mere allegations from the losing side, our democracy would enter a death spiral. We’d never see the whole nation accept the election again,” McConnell said.

As his supporters storm into the US Capitol in quickly-escalating protests, Donald Trump has broken his silence and tweeted for demonstrators to "support Capitol Police and Law Enforcement".

"They are truly on the side of our country," he wrote. "Stay peaceful!"

US media report that tear gas has now been used inside the Capitol building as a growing number of demonstrators enter the building.

DC Mayor Muriel Bowser has ordered a citywide curfew beginning at 1800 EST.