The US state of Pennsylvania's supreme court dismissed another legal challenge to the election by supporters of President Donald Trump on Saturday, further reducing his already near-impossible odds of overturning the results.

A Republican lawsuit had sought to invalidate mail-in ballots in the battleground state that President-elect Joe Biden won by about 81,000 votes -- or to throw out all votes and allow the state's legislature to decide the winner.

The court dismissed both claims in a unanimous decision, calling the second one an "extraordinary proposition that the court disenfranchise all 6.9 million Pennsylvanians who voted in the general election."

The lawsuit argued that a Pennsylvania law from 2019 allowing universal mail-in voting was unconstitutional.

The judges said that their November 21 challenge to the law was filed too late, coming more than a year after it was enacted and with the election results "becoming seemingly apparent."

Pennsylvania officially certified Biden's victory there on November 24. The lawsuit had also sought to stop certification.

Saturday's decision follows a long line of similar ones, including a ruling the day before in which a federal appeals court flatly dismissed Trump's claim that the election was unfair and refused to freeze Biden's win in Pennsylvania.

Trump has refused to give up on his claims of fraud in the November 3 election despite his repeated court defeats, tweeting bizarre conspiracy theories and vowing to continue his legal fight.

On Thursday, he said for the first time that he would leave the White House if Biden is officially confirmed the winner by the Electoral College on December 14.

But on Friday he tweeted that "Biden can only enter the White House as president if he can prove that his ridiculous '80,000,000 votes' were not fraudulently or illegally obtained."

Biden, who is to be sworn in on January 20, won 306 votes in the Electoral College to Trump's 232.

The president-elect has said that Americans "won't stand" for attempts to derail the vote outcome.