Republican support for Donald Trump’s defiance of the US presidential election result appears to be collapsing after Senate Majority Leader Mitch McConnell finally acknowledged Joe Biden as the winner.

Mr McConnell ended his long silence on the presidential election result on Tuesday (US time), congratulating Mr Biden and vice president-elect Kamala Harris in a speech to Congress.

“Our country has officially a President-elect and a Vice-President elect. Many millions of us had hoped the presidential election would yield a different result,” he said.

“But our system of government has processes to determine who will be sworn in on January 20. The Electoral College has spoken.”

It came a day after the US Electoral College formalised the Democrats’ win – and 38 days after it became clear Mr Trump was going to lose.

Mr Biden spoke to Mr McConnell late on Tuesday (US time) and said the pair would soon meet. Soon after, it was revealed that Secretary of State Mike Pompeo was to meet his likely successor in the Biden administration, Antony Blinken, and Republican Senator Lindsey, one of Mr Trump’s closest allies, said he’d spoken with some of Mr Biden’s cabinet picks.

Senate Majority Whip John Thune, also a Republican, appeared to sum up the growing sentiment earlier.

“In the end, at some point you have to face the music,” he said on Monday (local time), saying it was “time for everybody to move on” after the Electoral College vote.

Mr Biden said he spoke to Mr McConnell before heading to Georgia, where he will campaign for two Democratic Senate candidates whose January 5 run-offs could make or break his domestic policy agenda.

“While we disagree on a lot of things, there are things we can work together on,” Mr Biden.

“We agreed to get together sooner [rather] than later.”

Mr McConnell and his top deputies are also pressing other Senate Republicans not to join with any House Republicans who might object to the election outcome when Congress meets on January 6 to ratify the Electoral College vote.

Any such challenge is almost certain to fail since it would require approval by both chambers, including the Democratic-controlled House of Representatives.

Mr McConnell (right) with Mr Biden (left) and then US president Barack Obama in 2016. Photo: Getty

Also on Tuesday, Mr Biden was congratulated by Russian President Vladimir Putin, a favourite of Mr Trump’s, and Mexican President Andres Manuel Lopez Obrador.

Mr Biden and Ms Harris will be sworn in on the steps of the US Capitol on January 20 with far fewer people present than is customary due to the raging coronavirus that has killed more than 300,000 Americans.

In the mean time, Mr Biden has pressed ahead with building his cabinet.

He has reportedly chosen Pete Buttigieg, his former Democratic presidential nomination rival, to lead the Transportation Department.

Mr Buttigieg, 38, the former mayor of South Bend, Indiana, would be the first LGBTQ person nominated to Biden’s administration. If confirmed, he would also be the first LGBTQ cabinet secretary approved by the Senate.

US Representative Deb Haaland of New Mexico is said to be Mr Biden’s leading choice to head the Interior Department, a selection that would make her the first native American to lead a cabinet agency.

Mr Biden’s trip to Atlanta comes nine days after Mr Trump travelled to Georgia in support of the Republican senators seeking re-election.

Mr Biden’s narrow win in Georgia in November underscored the southern state’s transformation from Republican stronghold to competitive political battleground.

If the Republicans win either contest, they will maintain Senate control, allowing them to thwart many of Mr Biden’s legislative goals on issues such as the coronavirus pandemic, the economy and climate change.

A Democratic sweep would give Mr Biden control of the White House and both chambers of Congress.

-with AAP