President Trump on Friday claimed that President-elect Joe Biden must prove that the votes he received in the presidential election were not “illegally obtained” in order to enter the White House — the latest sign that Trump is not giving up his efforts to challenge the result.

Trump 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. When you see what happened in Detroit, Atlanta, Philadelphia & Milwaukee, massive voter fraud, he’s got a big unsolvable problem!”

Trump’s tweet came after Biden crossed the 80 million vote threshold as votes are still being counted. Fox News and other media outlets have projected that Biden has won the presidency. There is no requirement for Biden to “prove” his votes were legally obtained for him to take office.

Trump’s tweet is his latest remark to cast doubt on the election result, which he has challenged by alleging voter fraud in multiple swing states that he lost — specifically Pennsylvania.

His campaign has launched a number of legal challenges related to how votes were counted. But those challenges were dealt blows this week when multiple states certified their results.

He had asserted on Wednesday again that the election was stolen and said that “we can’t let that happen.”

“This election was rigged and we can’t let that happen. We can’t let it happen for our country,” Trump said via speakerphone to the Pennsylvania Senate Majority Policy Committee, “And this election has to be turned around because we won Pennsylvania by a lot and we won all these swing states by a lot.”

However, he said he would leave office if the Electoral College vote in December went to Biden

“Certainly I will. … You know that I will,” he said.

But regarding the series of legal challenges in multiple states, Trump said: “A lot of things [are] happening between now and Jan. 20.”