COLORADO SPRINGS • Warning darkly of a stolen election, Republican presidential candidate Donald Trump has called on supporters to turn out in droves on Nov 8 to monitor polling centres, saying they need to be vigilant against voter fraud and a rigged outcome.
"Voter fraud is all too common, and then they criticise us for saying that," he said at a rally in Colorado on Tuesday. "But take a look at Philadelphia, what's been going on, take a look at Chicago, take a look at St Louis. Take a look at some of these cities, where you see things happening that are horrendous."
His pronouncements drew a mocking rebuke from President Barack Obama, who suggested that Mr Trump's effort to delegitimise the election results even before the vote takes place was unprecedented. He told the Republican nominee to "stop whining and go try to make his case to get votes".
"I have never seen in my lifetime or in modern political history any presidential candidate trying to discredit the elections and the election process before votes have even taken place," Mr Obama said.
By turns mocking and grave, Mr Obama said the claims demonstrated that Mr Trump lacked the leadership or toughness to be president. But he warned that the charges would undermine the nation's purest expression of democracy, a popular vote respected by the vanquished as well as the victors.
Voter fraud 'extremely rare'
The type of in-person voter fraud that Republican presidential candidate Donald Trump has been warning about is extremely rare in the United States. One study by a Loyola Law School professor found 31 known cases of fraud out of a billion votes cast in US elections from 2000 to 2014.
Moreover, the ability to commit fraud on a scale vast enough to swing a statewide election would require the coordination of scores of people, a possibility widely dismissed by experts.
Voter fraud "does occur", said Mr Al Schmidt, a Republican who is the vice-chairman of Philadelphia's election board, who issued a report in 2012 that found a handful of irregularities.
"But what Trump's talking about is widespread, coordinated efforts to alter the outcome of the election, and that I've just simply never seen. It would be very easy to find because it would involve a conspiracy of dozens, if not hundreds, of people."
His sharp words reflected rising concerns among Democratic and Republican leaders that Mr Trump's drumbeat of accusations was resonating with his supporters. Many worry that if Mrs Hillary Clinton wins and Mr Trump refuses to accept the result, his stand will undermine her authority going into office and sow doubts about the legitimacy of the process.
There is no evidence, Mr Obama said, that a presidential election had ever been rigged. He said there was little indication that it could be, given that elections are run by the state and local authorities, with people from both parties supervising polling sites and ballot counting.
"The notion that somehow if Trump loses Florida, it is because of those people that you have to watch out for," he said, his voice thick with sarcasm. "That is both irresponsible, and by the way, doesn't really show the kind of leadership and toughness that you want out of a president.
"If you start whining before the game's even over, if whenever things are going badly for you and you lose, you start blaming somebody else, then you don't have what it takes to be in this job because there are times when things don't go our way - or my way."
There has also been criticism from many Republicans, including House Speaker Paul Ryan, the titular head of the party.
On Monday, Senator Marco Rubio of Florida, a primary challenger of Mr Trump's, joined in. "This election is not being rigged," he said, adding that Mr Trump "should stop saying that". He added: "We have 67 counties in this state, each of which conduct their own elections. I promise you there is not a 67-county conspiracy to rig this election."
Mr Trump's language has, however, stirred increasing fears of intimidation of minorities inside polling places, where their qualifications to vote could be challenged, or outside, where they would face illegal electioneering.
But as Mr Trump casts doubt on the integrity of the presidential election, there are no signs of a wave of Trump poll watchers building.
Republican and election officials in cities and states that Mr Trump has singled out for potential widespread voter fraud, including Philadelphia, Pittsburgh and Ohio, said his message to supporters to become poll watchers had generated scant response.
But even if few are heeding the tycoon's call to sign up as poll watchers, a big question is whether Trump supporters will nevertheless flood polling places on Election Day in Democratic strongholds.
Ms Lisa Deeley, a Democrat on the Philadelphia voting board, feared Trump supporters would gather at polling sites, where they are allowed to go within 3m of the entrances, to jeer voters. She said: "It's one thing for any candidate to say, 'I need volunteers, come out and support my campaign'. But when a candidate is saying, 'I need your help because they're cheating', it changes the game."
In general, states permit citizen poll watchers in polling locations to check the work of election officials. Qualifications differ by state.