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As Donald Trump takes bipartisan heat for his persistent claim the election could be “rigged,” the GOP nominee is holding up studies and reports of potential voter fraud to back up his case.

At his address in Gettysburg, Pa., on Saturday, Trump cited a 2012 Pew study that found evidence the voter registration rolls needed serious cleaning up.

The study found one in eight voter registrations were either invalid or inaccurate; more than 1.8 million dead people were listed as voters; and 2.75 million people were registered in more than one state.  

“Washington has tried so hard to stop our campaign,” Trump said. “The system is totally rigged and broken.”

The voter registration problems don't necessarily reveal any intentional plots to vote illegally. And in a follow-up report this month, Pew found states have implemented systems to clean up the voter rolls, including by comparing registration lists with other data sources to verify information.

But one citizen’s watchdog group, Respect Washington, said registration inconsistencies create the potential for fraud.

“There is a certain embedded inaccuracy and corruption within the voter registration rolls that can express itself as voter fraud come an election,” co-founder Craig Keller told Fox News.

Trump has faced criticism on the "rigging" claims not only from Hillary Clinton and the White House but some fellow Republicans.

“It’s irresponsible because you’re starting to undermine the public’s confidence in our elections,” Washington state Republican Secretary of State Kim Wyman told Fox News.

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However, Wyman noted there are gaps in voter registration security that can be exploited.

For instance, when people register to vote, they only declare they are U.S. citizens. Election officials cannot verify those claims, as proof of citizenship is not required to get a driver's license. Further, Washington state and seven other states are all out of compliance with the federal Real ID Act, which sets minimum standards for state-issued IDs.

Republican vice presidential nominee and Indiana Gov. Mike Pence has backed up his running mate, saying, “We have [voter fraud] in Indiana,” and pointing to a probe into a voter registration program in the Hoosier State.

Earlier this year, Texas Gov. Greg Abbott described voter fraud as “rampant.” Just this month, Abbott announced the “largest voter fraud investigation in Texas history” after an alleged scheme was uncovered involving up to 20,000 cases in which signatures for mail-in ballot applications did not match those on the actual ballots.

Meanwhile in September, a Colorado CBS affiliate found multiple cases of dead people casting ballots in elections, including one case where a woman who died in 2009 cast ballots in 2010, 2011, 2012 and 2013.

“We do believe there were several instances of potential vote fraud that occurred,” Colorado Secretary of State Wayne Williams said. “It shows there is the potential for fraud.”

Also in September, an investigation in Richmond, Va., was launched after 20 voter applications were turned in under the names of dead people.

“Often times we hear our Democrat colleagues suggest that voter fraud doesn’t exist in Virginia or is a myth,” House Speaker William H. Howell said, according to The Richmond Times-Dispatch. “Well it does indisputably exist.”

Fox News’ Dan Springer contributed to this report.