His reasoning included an implication of widespread voter fraud, asserting that there are "millions of people that are registered to vote that shouldn't be registered to vote."
But Trump is vastly overstating how common voter fraud is, according to election experts.
Voter fraud — in which a person casts a ballot despite knowingly being ineligible to vote — is "extraordinarily rare," according to a report by the Brennan Center for Justice at New York University Law School. The 2007 study examined elections where wrongdoing was alleged and found the rate of substantiated instances of fraud ranged between 0.00004% and 0.0009%.
Another study by a Loyola law professor found just 31 instances of in-person voter fraud (in which one person pretended to be someone else) out of more than 1 billion ballots cast between 2000 and 2014.
Trump has charged that individuals are voting multiple times in order to tilt the election away from him. But UC Irvine law professor Rick Hasen noted that such schemes are impractical to execute on a sufficiently large scale to sway the
"To vote five, 10 or 15 times one would have to either register five, 10, or 15 times in different jurisdictions or with false names or go five, 10 or 15 times to polling places claiming to be someone else whose name is on the voter rolls, in the hopes that this person has not already voted and you would not get caught," Hasen wrote this week.
Trump has also cited a 2014 study by Old Dominion University professors that asserted noncitizens illegally vote at high enough rates to affect election results. The study estimated that 6.4% of noncitizens voted in 2008 and posited that noncitizen votes could have been a determining factor in President Obama's victory in North Carolina that year. But that report has been widely criticized by experts for problems in its methodology.
Trump's allegations of a "rigged system" ignore the on-the-ground reality of how American elections are conducted. Elections are administered not nationally, but by states and localities — more than 8,000 jurisdictions in total. The top elections officers in pivotal battleground states, including Ohio, Nevada and Colorado, are Republicans.
Some of Trump's fellow Republicans are trying to downplay accusations of election fraud. GOP Sens. Marco Rubio of Florida and Pat Toomey of Pennsylvania, both in heated races in their bids for reelection, affirmed the legitimacy of the elections process.
"Our democracy relies on confidence in election results, and the speaker is fully confident the states will carry out this election with integrity," a spokesperson for Ryan said this week.
However, many voters, particularly Trump supporters, share in the GOP presidential nominee's cheating concerns. A Politico/Morning Consult poll found that more than 40% of voters — and nearly three-quarters of Republicans — think the election could be stolen from Trump because of fraud.