As part of their efforts, Trump and his allies promoted a "big lie" of numerous false claims and conspiracy theories claiming that the election was stolen by means of rigged voting machines, electoral fraud and an international communist conspiracy.p id="cite_ref-13" class="reference">[13]^[14][20] These allegations were dismissed as completely baseless by numerous state and federal judges, election officials, governors, and government agencies.^[21][22][23] United States Attorney General William Barr said on December 1 that U.S. attorneys and FBI agents had investigated complaints and allegations of fraud, and found no evidence of fraud at a large enough scale to influence the outcome of the election.^{[22][24][25][26]} Director of National Intelligence John Ratcliffe said no evidence had been found of foreign interference with voting tallies.^[27] Cybersecurity and Infrastructure Security Agency (CISA) Director Chris Krebs said there was no evidence that election systems had been compromised, refuted baseless claims that some vote-counting had taken place outside the United States, and called the election "the most secure in American history", leading Trump to fire him^[28] and Trump attorney Joseph diGenova to call for his execution.^[29][30][31]

Hundreds of elected Republicans, including members of Congress and governors, refused to acknowledge Biden's victory,^[32] though some did so after the certification of the Electoral College vote.^[33][34] Emily Murphy, the administrator of the General Services Administration, delayed the start of the presidential transition before allowing it to proceed on November 23, 20 days after the election and 16 days after most media outlets had projected Biden to be the winner.^[35][36]

Trump and his allies encouraged federal and state officials, particularly in states that Biden won, to throw out legally-cast ballots, challenge vote-certification processes, and overturn certified election results.^[37][38] He pressed Justice Department leaders to challenge the election results and publicly state the election was corrupt.^[39][40][41] In an early January 2021 phone call, he pressed the Georgia secretary of state to "find" the 11,780 votes needed to secure his victory in the state.^[42] He repeatedly urged Georgia Governor Brian Kemp to convene a special session of the legislature to overturn Biden's certified victory in the state; and made a similar plea to the Pennsylvania Speaker of the House.^{[10][43][44][45]} On a conference call, he asked 300 Republican state legislators to seek ways to reverse certified election results in their states.^[46]

Before and after the election, Trump said he expected the outcome would be decided by the Supreme Court, where conservative justices held a 6–3 majority, with three of the justices having been appointed by Trump.^[47][48] After the election, his legal team sought a path to bring a case before the Court. The Trump campaign and its allies filed at least 63 lawsuits on and after Election Day,^[49][50][51] none of which were successful.^[50][52] On December 10, 2020, after several lawsuits had been dismissed, Trump tweeted, "This is going to escalate dramatically. This is a very dangerous moment in our history...The fact that our country is being stolen. A coup is taking place in front of our eyes, and the public can't take this anymore."^[53] The next day, the Supreme Court declined to hear Texas v. Pennsylvania, a case hailed by Trump as "the big one" and supported by numerous Republicans,^[54] which sought to overturn the results in Pennsylvania, Michigan, Georgia, and Wisconsin, all swing states won by Biden.^[55] Five lawyers who represented Trump resigned in January 2021 after claiming he coerced them to repeat false claims of voter fraud.^[56]

After the failure of Texas, Trump reportedly considered additional options, including military intervention, seizing voting machines and another appeal to the Supreme Court, as well as challenging the congressional counting of the electoral votes on January 6, 2021.^[57][58] Former Trump national security advisor Michael Flynn, who received a presidential pardon shortly after the election, had called on the president to suspend the Constitution, silence the press, and hold a new election under military supervision.^[59]

By December 30, multiple Republican members of the House and Senate indicated their intent to object to the congressional certification of Electoral College results in order to force both chambers to debate and vote on whether to accept the results.^[60][61][62] Mike Pence, who as vice president would preside over the proceedings, signaled his endorsement of the effort, stating on January 4, "I promise you, come this Wednesday, we will have our day in Congress." Trump and some supporters promoted a false "Pence card" theory that the Vice President has the authority to reject certified results.^[63][64][65]

As Congress convened to certify the results, Trump held a rally on the Ellipse. He then encouraged his supporters to march to the Capitol building, which they attacked. Five people died either shortly before, during, or shortly after the attack: one was shot by Capitol Police, one died of a drug overdose, and three succumbed to natural causes. Over 100 police officers were injured.^[66] One week later, Trump was impeached a second time for incitement of insurrection but was acquitted by the Senate. Trump did not attend Biden's inauguration.

Trump's attempts to overturn the election cost taxpayers $519 million, according to an analysis by The Washington Post, including $488 million in security fees during and after the 2021 United States Capitol attack and $2.2 million in legal fees.^[67] As of November 2021, Trump along with many members of the GOP continue to insist that the election was stolen.^[68][69][70]