More than 80 US courts looked into the allegations of fraud in last year's elections but found nothing.
President Joe Biden has said that protecting voting rights from what he called Republican "election subversion" poses a historic "test" for the United States in the wake of the 2020 election.
"It's up to all of us to protect that right. It's a test of our time," he said in a speech on Tuesday in Philadelphia, birthplace of the US Constitution.
Referring to attempts led by Donald Trump to overturn the result of the presidential election, as well as pile on new voting rules in the name of security, Biden said "this is election subversion."
"It's the most dangerous threat to voting and the integrity of free and fair elections in our history," he said in a fiery speech.
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Biden noted that in response to Republican allegations of fraud in 2020, more than 80 courts — right up to the Supreme Court — held hearings and in every case found nothing significantly wrong.
The 2020 presidential and congressional election saw the highest turnout in history, despite the Covid-19 pandemic, and was subsequently "the most scrutinised election ever in American history," he said.
Without naming Trump, he made clear what he thought of the Republican's unprecedented campaign to overturn the results.
"The big lie's just that: a big lie," he said.
"In America if you lose you accept the results, you follow the constitution. You try again. You don't call facts 'fake' and then try to bring down the American experiment just because you're unhappy."
Targeting non-white voters
The speech in the city where the founding document of the US democratic system was drawn up and signed in 1787 is Biden's highest profile foray into a controversy that both Republicans and Democrats describe in dire terms.
According to the White House and congressional Democrats, Republicans are using state legislatures to restrict voting rights across the country under the guise of increasing election security.
Republicans, egged on by Trump, insist tougher voting rules are needed to crack down on voter fraud.
This means things like cutting back on mail-in voting, shortening hours at polls and imposing heavy fines against poll workers who make mistakes.
While Republicans say such measures would clean up US elections, Democrats point to an already extremely low incidence of fraud and say the measures target Black and other non-white voters, who tend to vote Democratic.
"They want to make it so hard and inconvenient that they hope people don't vote at all," Biden charged.