PHILADELPHIA -- In Wednesday's third and final presidential debate, Donald Trump made history by becoming the first major party candidate to refuse to say whether he would honor the election's outcome if he loses.

A day later at a rally in Ohio, he told supporters he would accept "a clear election result" but would reserve his right "to contest or file a legal challenge in the case of a questionable result."

Trump didn't say what might qualify as a "questionable result." But he's made it clear that he already thinks the election is rigged against him. It's almost universally agreed that is a virtual impossibility. Unfortunately, the electronic voting machine millions of Americans will use to cast their ballots can be rigged, and thanks to outdated technology it will be difficult to prove they weren't if Trump or his supporters put forth such a claim.

Verified Voting, a nonprofit group dedicated to providing information on elections, said eight out of ten Americans will cast their ballot this year on an electronic voting machine that produces some form of hard copy record of their vote.

But that leaves over a dozen states in this election cycle using a direct recording electronic (DRE) machine -- often a button-based or touchscreen device used for recording vote counts -- which don't support paper audit technology. In several key battleground states, electronic voting machines with paper audit trails are virtually non-existent.

In Georgia, where the latest polls show Clinton and Trump deadlocked for 16 electoral votes, more than six million registered voters will cast their ballot on a voting machine that lacks a voter-verified paper audit.

But by far the biggest concern this year is Pennsylvania, where the candidates will vie for 20 electoral votes.

Pennsylvania has ten different voting machine models, but by far the most popular -- used in 25 counties and the city of Pittsburgh -- is the ES&S iVotronic voting machine. As many as three million Pennsylvania voters will cast their ballot on an iVotronic system, which records its votes on internal flash memory leaving no hard copy record.

The iVotronic system has been singled out as one of the most problematic voting devices currently in use.

According to court records the the machine has lost votes, registered "phantom" votes, counted votes twice, inaccurately tabulated votes, switched votes, reached its capacity and started counting backwards, and recorded wrong votes during elections in other states.

And, computer scientists at the University of Pennsylvania found that the machines are easy to tamper with.

"We identified numerous critical vulnerabilities in nearly every component of the ES&S system," the researchers said. "Virtually every mechanism for assuring the integrity of precinct results and for protecting the back-end tallying system can be circumvented."

For instance, they said, a voter can surreptitiously delete all vote data and audit logs stored on a touchscreen voting machine using a Palm handheld device and a small magnet.

Worse still, a poll worker uses a device called a personal electronic ballot to activate and deactivate individual iVotronic stations, which one expert surmised could make it possible to transmit a corrupted file from one machine to another.

For weeks, Trump has singled out Pennsylvania for scrutiny. He has gone so far as to call on his followers to act as poll watchers in precincts in Democratic strongholds like Philadelphia, raising the specter of voter intimidation.

In 58 of the state's 67 counties, including those comprising its two largest cities -- Philadelphia and Pittsburgh -- once a vote is cast, it exists only in electronic format. That means if the election is close -- and Clinton fails to sweep the state by a landslide margin -- Trump's claim that the system is "rigged" will be virtually impossible to disprove.

State and local officials in Pennsylvania have denounced Trump's claims of election rigging and have vigorously defended the integrity of the state's electoral process.

But Trump won't have trouble finding support for his conspiracy theory from some of the very same public officials who now call his concerns unfounded -- including Pennsylvania's own deputy commissioner of elections, Marian Schneider.

Ten years ago, as a private attorney, Schneider joined in a massive bipartisan lawsuit challenging the integrity of electronic voting machines in the commonwealth.

In her lawsuit, filed in 2006, Schneider represented a diverse assortment of co-plaintiffs including the NAACP, the Public Interest Law Center, and other senior political officials, such as Rob McCord, the former state treasurer and a recent candidate for governor.

The plaintiffs argued that the state's voting machines violate the Pennsylvania Election Code and Constitution, and have "eroded the public's confidence in the election process because there can be no audit or effective recount following machine failure of accusations of unreliability or tampering."

"The certified DREs have repeatedly malfunctioned in Pennsylvania and in other states, and are known to be vulnerable to malicious tampering," they said in their complaint, "all of which underscores the importance of the Election Law's requirement of a permanent record of each voter's vote that can be used to audit the accuracy of the results reported by the machines."

For nearly a decade the case moved through the courts, while a number of other states across the country -- moved by the same concerns -- abandoned DRE voting machines entirely or else mandated that they produce an individual paper audit.

The Pennsylvania Supreme Court dismissed the suit last year. By then Schneider had taken her post in the administration of Pennsylvania's new governor Tom Wolf.

Only last week, she said the state is doing everything in its power to ensure the integrity of its voting machines.

"We have had conversations with Homeland Security regarding the services they could provide before November," said Schneider in a statement. "Our goal is to proactively take any steps possible to make sure that Pennsylvania's statewide election infrastructure is as secure as possible."

In the end it may not even matter if the machines are actually compromised, though. Trump has been busy convincing his followers that losing Pennsylvania is all the proof they need that the system is rigged.

Under Pennsylvania law a recount is mandated if a candidate loses by a margin of half-a-percentage point or less. Trump can't personally request a recount if he loses by a greater margin. (In that case he would have to go to court to challenge the election's outcome). But his supporters can. All it takes is three voters and $50 to file a petition for a recount in each precinct. With more than 9,000 precincts in the state, it's hard to imagine he'd leverage the support needed to effectuate a statewide recount, but all it takes is one contested precinct to delay an election's outcome by weeks.

That's a prospect that keeps election watchers up at night.

"What do we do if, at the end of the election, it comes down to Pennsylvania and there's a challenge saying, you know, these machines were corrupted?" said Avi Rubin, a computer security expert at Johns Hopkins University, in a recent interview.

"We can't do recounts. We don't have paper ballots," said Rubin. "We just have to live with those machines."

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