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.
The decision means Mr Biden is able to receive top security briefings and access key government officials and millions of dollars in funds as he prepares to take over on 20 January.
Asked on Thursday whether he would agree to leave the White House if he lost the electoral college vote on 14 December, Mr Trump responded: “Certainly I will, certainly I will and you know that.”
However, he went on to say that “if they do [elect Joe Biden], they made a mistake”, adding: “It’s going to be a very hard thing to concede.”
He also made more claims of “massive fraud” during the election, without offering any proof.
Under the US electoral system, voters do not directly choose the next president.
Instead, they vote for 538 officials, who are allocated to American states based on their population size.
The electors almost always vote for the candidate that won the most votes in their states, and although it is possible for some to disregard the voters’ pick no result has ever been changed this way.