Washington - Donald Trump's assertion that the US presidential election will be rigged has triggered alarm among observers, who warn he could be laying the groundwork for rejecting a Hillary Clinton win and even sparking civil unrest.The Republican 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 on Monday, warning to a crowd in Columbus, Ohio that the general election itself - already filled with bitter partisan rancour - 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 some experts say his 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 said.'Stoking fear'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 Centre study tracking public confidence in the accuracy of the national vote.In 2004, 48% of Americans said they were very confident that the votes across the country were accurately counted, Pew reported. That number slipped to 43% in 2008 and just 31% in 2012.Voter fraud has played a role throughout the history of the republic, including the very close 1960 race between John F Kennedy and Richard Nixon, when votes were apparently cast by the dead in Illinois. Voter controversies, including accusations of access restrictions, swelled in Ohio in 2004.Fuelling voter scepticism among the average voter are sets of rules that differ from state to state."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, and 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.Won't go quietlyDemocrats, 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 that "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 and effectively handed the presidency to George W Bush, Democrat Al Gore conceded defeat 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.