President Donald Trump said Friday that Democratic presidential nominee Joe Biden could only enter the White House as president if he could prove that the millions of votes he received in the election were not illegally obtained.

Trump issued the statement less than a day after saying that he would leave the White House should Biden be voted in the electoral college.

On Friday, Trump tweeted that the former vice president can only enter the White House as new president if he could 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!" he added.

This was the latest sign that Trump is not giving up his efforts to challenge the election result. Biden's votes have already reached 80 million as official election results are still being counted, Fox News reported.

Related story: Trump Files Lawsuits, Laying Ground for Contesting Election Results

Fox News and other media outlets have already forecasted that Biden has won the presidential seat.

Reports said that Biden does not need to prove his votes were illegally obtained to take office. Trump also does not have the authority to determine whether the former vice president can take office.

Trump has challenged the election result by alleging voter fraud in several key swing states he lost, particularly Pennsylvania.

The Trump campaign has initiated numerous legal charges concerning how votes were counted. But the Trump campaign failed to prove fraud or misconduct that would justify the remedies they have sought since Election Day. Several states have already announced the election results.

Trump reiterated on Wednesday that the election was being stolen from him. He claimed that the election was rigged, and people cannot let that happen.

Trump added that this election has to be turned around because he won Pennsylvania "by a lot," as well as in other swing states. He further noted that a lot of things can still happen between now and Jan. 20.

Earlier this week, the General Services Administration (GSA) has already provided the Biden team with access to agencies, federal office space, and funds.

GSA Head Emily Murphy said she decided on her own based on the law and available facts. Murphy was a Trump appointee.

Recent developments, legal challenges, and certification of results were among the factors Murphy considered in making her decision.

Trump has not yet conceded despite several legal blows and certification of election results in some key swing states, which includes Michigan on Monday.

Biden's team should now be allowed government office space, as well as meetings with the Trump administration, according to an NPR report. The team will also receive around $7.3 million to pay staffers and other expenses.

As stated under the 1963 Presidential Transition Act, the GSA head would make the ascertainment even though the law is vague on the criteria that should be used.