Donald Trump has said he will leave the White House if Joe Biden is formally confirmed as the next US president by the electoral college.
The president has refused to accept defeat in the 3 November vote, and told reporters on Thursday it would be “hard” to concede.
He also once again repeated unsubstantiated claims of voter fraud.
Mr Biden leads Mr Trump 306 votes to 232 under the electoral college system used to pick US presidents.
The tally is far more than the 270 needed to win, and Mr Biden also leads the popular vote by more than six million.
Electors will meet next month to formalise the vote, with the Democrat Joe Biden due to be sworn in as president on 20 January.
The president and his supporters have lodged a number of legal challenges over the election, but most have been dismissed.
Earlier this week, Mr Trump finally agreed to allow the formal transition to President-elect Biden’s team to begin, following several weeks of uncertainty.