Speaking to reporters on the Thanksgiving holiday, Republican Trump said if Democrat Biden - who is due to be sworn in on January 20 - is certified the election winner by the Electoral College, he will depart the White House.
Asked if he would leave the White House if the Electoral College votes for Biden, Trump said: "Certainly I will. Certainly I will. And you know that."
But Trump said it would be hard for him to concede because "we know there was massive fraud."
"It was a rigged election ... at the highest level," Trump insisted in a sometimes rambling discourse at the White House, while continuing to offer no concrete evidence of widespread voting irregularities.
It was the first time Trump has taken questions from reporters since Election Day, and at times turned combative.
In the United States, a candidate becomes president by securing the most “electoral” votes rather than by winning a majority of the national popular vote. Electors, allotted to the 50 states and the District of Columbia largely based on their population, are party loyalists who pledge to support the candidate who won the popular vote in their state.
Biden won the election with 306 Electoral College votes - many more than the 270 required - to Trump's 232, and the electors are scheduled to meet on December 14 to formalise the outcome. Biden also leads Trump by more than 6 million in the popular vote tally.
Trump has so far refused to fully acknowledge his defeat, though earlier this week - with mounting pressure from his own Republican ranks - he agreed to let Biden's transition process officially proceed.
Frenzied efforts by Trump and his aides to overturn results in key states, either by lawsuits or by pressuring state legislators, have failed, and he is running out of options.
“President-elect Biden won 306 electoral votes. States continue to certify those results, the Electoral College will soon meet to ratify that outcome," Michael Gwin, a spokesman for the Biden campaign, said when asked about Trump's comments. "Biden will be sworn in as President on Jan. 20, 2021.”
Showing that he intends to stay in the political fray until the end of his term, Trump said on Thursday he would travel on December 5 to Georgia, a once solidly Republican state he lost narrowly to Biden, to campaign for two Republican US Senate candidates.
The two runoff elections in Georgia on January 5 will determine whether the Republicans keep their majority in the Senate.
"I just want to tell my people: don't be disappointed yet because this race is far from over," Trump said.
BIDEN, AND TRUMP STAY CLOSE TO HOME AMID PANDEMIC
Biden and Trump both stayed close to home to celebrate Thanksgiving on Thursday as the coronavirus pandemic raged across the country.
Biden spent the holiday in the small seaside town of Rehoboth, Delaware, where he and his wife Jill have a holiday home. The Bidens are hosting daughter Ashley Biden and her husband Dr Howard Krein for the holiday meal.
The former vice president, appearing with his wife in a video message posted to his Twitter account on Thanksgiving, said his family typically holds a large gathering on the island of Nantucket off Massachusetts, but would remain in Delaware this year "with just a small group around our dinner table" because of the pandemic.
In the presidential-style address to a nation that has lost more than 260,000 lives to the coronavirus, the Democratic president-elect said on Wednesday that Americans were making a "shared sacrifice for the whole country" and a "statement of common purpose" by staying at home with their immediate families.
"I know this isn't the way many of us hoped we'd spend our holiday. We know that a small act of staying home is a gift to our fellow Americans. I know better days are coming."
Trump often likes to celebrate holidays at his Mar-a-Largo resort in Florida. But on Thursday he remained in the Washington area, spending part of the morning at his Trump National Golf Club in Virginia where he played a round of golf.
It was a far cry from last year when he made a surprise visit to Afghanistan, where he served turkey to US troops before sitting down to eat Thanksgiving dinner with them.
This time, Trump spoke by video link from the White House to members of the military at home and abroad.
In contrast to Biden, who pleaded with Americans to celebrate the holiday safely by wearing masks and socially distancing, Trump in his Thanksgiving proclamation on Wednesday urged Americans to "gather" for the holiday.
On Thursday, the president tweeted "HAPPY THANKSGIVING!" in response to the US Supreme Court's decision late on Wednesday to block New York state from imposing attendance limits at religious services.
Governor Andrew Cuomo dismissed the ruling as "irrelevant," saying it related to specific areas that were no longer considered at high risk.
