"The numbers tell us ... it's a clear and convincing story: We're going to win this race," Biden said on Friday, adding that he and his running mate Kamala Harris were already meeting with experts as they prepare for the White House.

Biden's speech was originally planned as a victory celebration, but he changed his approach in the absence of an official call from TV networks and other election forecasters.

Still, it amounted to a blunt challenge to Trump, who kept out of view in the White House on Friday as Biden's lead grew in the four states that will decide the outcome: Pennsylvania, Georgia, Arizona and Nevada.

Leading Trump by 4.1 million votes nationwide out of a record 147 million cast, Biden said Americans had given him a mandate to tackle the coronavirus pandemic, the struggling economy, climate change and systemic racism.

"They made it clear they want the country to come together, not continue to pull apart," Biden said.

He said he hoped to address Americans again on Saturday.

Trump has remained defiant, vowing to press unfounded claims of fraud as his Republicans sought to raise $US60 million ($A82 million) to fund lawsuits challenging the results.

But some in his camp described the legal effort as disorganised, and so far they have not found success in the courts.

On the fourth day of vote counting, former vice president Biden had a 253-to-214 lead in the state-by-state Electoral College vote that determines the winner, according to Edison Research.

Securing Pennsylvania's 20 electoral votes would put Biden over the 270 he needs to win the presidency after a political career stretching back nearly five decades.

Biden would also win if he prevails in two of the three other key states. Like Pennsylvania, all three were still processing ballots on Friday.

Australian Associated Press