The sharp reaction came amid alarm among observers who warn that Trump could be laying the groundwork for rejecting a Clinton win
President Barack Obama on Thursday dismissed as "ridiculous" Republican White House candidate Donald Trump's assertion that the US election in November will be rigged in Democrat Hillary Clinton's favor.
"If Mr. Trump is suggesting there is a conspiracy theory that is being propagated across the country, including in places like Texas, where it is typically not Democrats in charge of voting booths, that is ridiculous," Obama told a press conference at the Pentagon.
The sharp reaction came amid alarm among observers who warn that Trump could be laying the groundwork for rejecting a Clinton win and even sparking post-election civil unrest.
Trump has routinely claimed that Bernie Sanders lost his Democratic primary race against Clinton because of a "rigged system" packed with superdelegates who could vote for the candidate of their choice.
But he took his complaints to a new level Monday, warning a crowd in Columbus, Ohio that the general election itself -- already filled with bitter partisan rancor -- will be marred by corruption.
That same night he sounded the alarm again: "I'm telling you, November 8th, we'd better be careful, because that election is going to be rigged," the provocative billionaire told Fox News. "And I hope the Republicans are watching closely, or it's going to be taken away from us."
Trump's calling card is raw authenticity, and his campaign has drawn millions of voters precisely because he has attacked the current political system and the Washington establishment as corrupt.
But while Obama noted how competitors in the schoolyard or sports field complain of being cheated when they lose, "I have never heard of someone complaining of cheating... before the score is tallied."
Some experts say Trump's remarks threaten the bedrock principle that America's presidential election is peacefully contested regardless of the political turmoil roiling the country.
"Statements like those that Trump made, without any explanation, it does undermine the legitimacy of the system," Michael Heaney, an assistant professor of political science at the University of Michigan, told AFP.
But while Heaney said Trump's rigged election remarks are "part of his pattern of stoking fear," the nominee is tapping into legitimate public concerns about America's complex and even arcane election process.
Trust in that system has waned, according to a Pew Research Center study tracking public confidence in the accuracy of the national vote.
In 2004, 48 percent of Americans said they were very confident that votes were accurately counted, Pew reported. That number slipped to 43 percent in 2008 and just 31 percent in 2012.
Voter fraud has played a role throughout the years, including in the close 1960 race between John F Kennedy and Richard Nixon, when votes were apparently cast by the dead in Illinois. Voter controversies swelled in Ohio in 2004.
Rules that differ from state to state fuel skepticism among average voters.
"We actually have something like 50 election systems rather than a unified election system," said Lorraine Minnite, professor at Rutgers University and author of "The Myth of Voter Fraud."
Nevertheless, general election fraud remains exceedingly rare, experts say and studies show. Minnite stressed it was "not common" for a presidential candidate to suggest an upcoming election will be rigged.
Republican politicians and officials have regularly pressed for a toughening of voter identification laws.
The threat of fraud "has been used to justify the rules that we've seen a revival of in the last several years that are actually making access more difficult for certain groups of people," Minnite said.
Democrats including Clinton have warned that Republicans seek to impose such rules to suppress the Democratic vote.
Pamela Smith, president of the non-partisan election monitor Verified Voting, said "the trend has been towards more paper, physical ballots" and away from unverifiable types of systems in order to ensure accountability and accuracy.
Accusations that a person illegitimately won the election are not new, but the possibility of a loser in the race making hay may be close to unprecedented.
In 2000, after a legal battle halted the manual recount of ballots in Florida, Democrat Al Gore conceded defeat to George W. Bush while never rejecting the outcome.
Part of Trump's not-so-subtle messaging this week was to suggest he will not go quietly should he fall to Clinton in November.
While Trump's free speech is protected by the US Constitution, "I also think there are real worries about an invitation to violence or other kinds of mob action," said Ohio State University professor Dan Tokaji, who teaches election law.
Presidential losers throughout US history have largely been gracious in defeat, "but Donald Trump is already intimating he will not be similarly gracious should he lose," Tokaji said.