Trump does not have the power to determine whether Biden can take office, and his campaign has failed to prove any mass fraud in court.

In a Friday tweet filled with conspiracy theories and lies, President Donald Trump implied he will not allow President-elect Joe Biden to take office, which he does not have the power to do, unless Biden proves that his election win was not fraudulent. 
“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 tweeted, invoking false and racist tropes of election fraud and malfeasance in four heavily Black cities located in states that voted for Biden. 
Biden does not have to disprove mass fraud to become president, and there is no evidence of widespread voter fraud in the 2020 election.
Biden won 306 Electoral College votes compared to Trump’s 232, and he won the popular vote by four percentage points, 51% to 47%, according to Insider and Decision Desk HQ. He is the first presidential candidate in history to win over 80 million raw votes. 
Georgia, Michigan, and Pennsylvania have already certified their presidential election results after a statewide risk-limiting audit Georgia and multiple failed legal challenges from the Trump campaign in Michigan and Pennsylvania. The Trump campaign has requested recounts in the entire state of Georgia and in two Wisconsin counties, Milwaukee and Dane, prior to statewide certification, both which are expected to affirm Biden’s victories in the states.
There is no evidence of widespread voter fraud in any of the cities listed in Trump’s tweet. In the three-and-a-half weeks since Election Day, neither the Trump campaign nor its allies have proved any level of fraud, misconduct, or malfeasance that would justify the remedies they have sought, including halting the counting of ballots, getting large numbers of ballots disqualified, and halting the formal certification of election results. 
Many of the lawsuits filed have not even claimed there is significant fraud at all, but have sought to halt ballot counting and certification over disputes around the access election observers were given to watch the ballot-counting process and officials’ treatment of mail ballots.  
Furthermore, Trump actually improved on his 2016 performance in some cities, including Philadelphia, while Biden made most of his gains in the suburbs, as outlets including Politico and the Washington Post have noted. 
Trump spent the Thanksgiving holiday perpetuating nonsensical and often contradictory claims of voter and election fraud on Twitter and at bill signing ceremony at the White House, where he also said he would not concede but would voluntarily leave the White House if the Electoral College votes to make Biden president in December. 
Trump himself does not have the authority to determine whether Biden “can enter” the White House. After states certify their election results, a currently ongoing process, the slates of presidential electors selected in each state will formally vote on December 14. 
And after a weeks-long stalemate, the General Services Administration (GSA) has formally ascertained Biden as the president-elect, kicking off the official transition process and sharing of resources between the outgoing and incoming administrations. 
Trump’s campaign is continuing to raise money and ask for donations (a substantial portion of which are going to paying off the campaign’s debts and to Trump’s new leadership PAC) as he refuses to conced and push discredited allegations of election fraud.