President Donald Trump said on Friday that Democratic nominee Joe Biden can only enter the White House as the next commander-in-chief if he is able to prove votes were not “fraudulently or illegally obtained.”

“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 wrote in a Twitter post.

The post is the latest indication that Trump is not giving up his efforts to challenge the results of the 2020 general election in several battleground states. The Trump campaign is facing an uphill battle in its legal challenges filed in Pennsylvania and Michigan aimed at protecting the integrity and accuracy of elections. The campaign has also expressed support for lawsuits filed in other battleground states challenging the Nov. 3 results.

Meanwhile, former federal prosecutor Sidney Powell is representing Trump-pledged presidential electors in Georgia and Michigan in challenging election results in both states over allegations of “massive fraud” in particular ballot stuffing and voter manipulation through the use of the Dominion voting system. Trump had previously announced that Powell was part of the campaign’s team but in a later statement the campaign said Powell was “practicing law on her own.”

Trump and his campaign have been vocal over the need to protect the sanctity of the ballot box while claiming that Democrats are trying to steal the election from him due to the counting of late-arriving ballots, which he alleges are illegal. He and his legal teams have been arguing that mail-in ballots postmarked by Nov. 3 but received after election day should not be counted and that votes that were counted without Republican observers present in the ballot-counting centers should also be considered illegal votes.

The president has also been claiming that the results have been tainted by irregularities and voter fraud. Trump’s personal lawyer Rudy Giuliani, who is leading the campaign’s efforts in the challenges, has claimed that there was a broad effort by Democrats in battleground states to steal the states for Biden.

He asserts that the campaign has at least 10 witnesses who are ready to testify about voter fraud in court but can’t yet be revealed publicly because doing so would put their lives in jeopardy. He also claimed that he has at least 1,000 sworn affidavits from citizens alleging wrongdoing that is “enough to overturn any election.”

“If we are going to present things in court, if we present it to you, judges are not going to be very happy with us. And finally, I have to tell you, our witnesses don’t want to be exposed to the tender mercy of a vicious press,” he said.

While some cases are still ongoing, a number of Trump campaign cases filed in those states have been dismissed by judges or withdrawn. Many third-party cases have also been filed but face similar challenges.

On Friday, the Third Circuit of Appeals in Pennsylvania rejected President Donald Trump’s campaign appeal in their challenge of the state’s election results. The panel ruled that the Trump campaign’s challenge doesn’t have merit, saying that “voters, not lawyers, choose the President. Ballots, not briefs, decide elections. The ballots here are governed by Pennsylvania election law.”

Trump lawyers Jenna Ellis and Giuliani responded to the ruling, saying that the case was headed to the Supreme Court.