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Republican presidential nominee Donald Trump has alleged that "large scale voter fraud" is occurring in the US, but is it possible to rig the US election?

The Trump campaign has made claims of "election rigging" for months, blaming the "dishonest and distorted media" and the "Clinton machine" for the Republican's slide in battleground states in the polls.

But now the rhetoric has reached new heights, with Mr Trump launching a twitterstorm to hammer home his allegations and declining to say, in his third debate, that he would accept the result if he loses.

So what are the claims, and how do they stack up?

1. Voter fraud

Media playback is unsupported on your device Media caption US Election 2016: Trump fans discuss 'election rigging'

Mr Trump's assertion that "large scale voter fraud" is happening is arguably Donald Trump's most serious allegation.

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He's not alone in this sentiment. Only a third of Republicans say they have a great deal or quite a bit of confidence that votes will be counted fairly, according to the Associated Press-NORC Center for Public Affairs Research.

However, studies suggest voter fraud isn't a widespread problem in the US.

In 2014, Justin Levitt, a professor at Loyola Law School, found 31 known cases of impersonation fraud in one billion votes cast in all US elections between 2000 and 2014.

And in 2012, News21 analysis of 2,068 alleged election-fraud cases since 2000 turned up 10 cases of voter impersonation.

The idea that the US election will be rigged is "ludicrous", and "certainly not stolen in the way that Trump has alleged," according to Professor Richard Hasen, an expert in election law.

"We cannot have thousands of people voting 5, 10, or 15 times as he's suggested without it being detected," he says.

Image copyright Alamy Image caption Voter ID has been a very contentious issue in the US

Commentators point out the US election administration is highly de-centralised, with each state setting its own rules and local officials administering them. In most states, observers keep tabs on poll workers too.

Voter ID requirements and voting machines also have huge variations, so widespread rigging would be hard to co-ordinate.

Ohio Secretary of State Jon Husted, a Donald Trump supporter, told CNN: "I can reassure Donald Trump, I am in charge of elections in Ohio and they're not going to be rigged.

"It's bipartisan, it's transparent, and there's just no justification for concern about widespread voter fraud."

2. 'Dead people generally vote for Democrats'

Image copyright Reuters Image caption Mitt who?

On Sunday, Donald Trump's adviser Rudy Giuliani told CNN he'd have to be a "moron" to say he thought the Philadelphia and Chicago elections would be fair, suggesting American cities, controlled by Democrats, are hotbeds of voter fraud.

He also said he recalled 720 dead people voting in the 1982 election in Chicago, and "dead people generally vote for Democrats instead of Republicans".

Lloyd Hitoshi Mayer, a professor of law at the University of Notre Dame, says every election cycle sees a handful of allegations that people voted in the name of a dead person, but "the numbers are very, very small".

Philadelphia came under scrutiny in 2012 after it emerged Republican presidential candidate Mitt Romney didn't receive a single vote in 59 voting divisions in the city.

The Republican Party looked into the allegations and found nothing irregular. It also transpired there were precincts in Utah that only voted for Mitt Romney.

One Philadelphia elections inspector didn't take kindly to the allegations resurfacing on Fox News recently - refuting the claims in a tweetstorm of 23 tweets.

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3. People are bussed in to vote several times

Mr Giuliani also said some people voted eight or 10 times when he ran for the mayor of New York City in 1989, because of "bussing people in from Camden".

It is common for voting rights groups and political campaigns to provide transport to take people to the polls.

Buses often target areas where residents are more likely to support a particular candidate or political party.

Image copyright Getty Images Image caption A charter bus waiting to take people to vote in 2012

Mayer says although there are sometimes allegations people are bussed to multiple polling stations in order to vote multiple times, he is not aware of any significant number of prosecutions or convictions for voter fraud based on such tactics.

4. 'Stolen' votes

Former House Speaker and Republican Newt Gingrich harked all the way back to the Richard Nixon versus JFK 1960 election this weekend, saying no "serious historian doubts that Illinois and Texas were stolen".

His comments refer to allegations that JFK's operatives - allegedly with the collusion of public officials - fixed tallies in Texas and Illinois, giving him 51 electoral votes, and ultimately winning him the closely contested election.

Douglas Brinkley, a professor of history at Rice University, told CNN it's still "undetermined issue" among historians.

"There were actually recounts in Illinois in 1960. And they didn't find that there was enough fraud to throw the election," he said. "In Texas it's also unclear that it threw the election."

Voter suppression has gone on throughout history, notably in the South in the 1950s and 60s, says Brinkley, but Trump's claim of election rigging now is "bogus and anti-historical".

Professor Mayer says the ability of law enforcement authorities, political parties, and the media to monitor such activities in 2016 is "much, much greater", so a repeat of "such 'stealing' of a presidential election, even if it occurred, would be impossible".

And, if election rigging did occur...

Image copyright Reuters Image caption A poll worker checks ID

The irony with the Trump campaign's remarks about election rigging is most of them suggest there will be in-person voter impersonation on election day, which studies show is the rarest form of voter fraud, according to Professor Mayer.

He says the most common forms of voter fraud are election official fraud - either in the form of stuffing ballot boxes, or "losing" ballots - and absentee ballot fraud.

But even these are becoming increasingly rare, largely because jurisdictions have adopted relatively easy ways to combat them, he adds.