The GOP nominee has specifically targeted Pennsylvania as a state where the election may be “stolen,” despite no evidence to back up such a claim and several polls showing Democratic rival Hillary Clinton well ahead of him here.

“The only way we can lose,” he told a recent rally in Altoona, “is if cheating goes on.”

But Andrew Appel, a Princeton professor of computer science, said that given a screwdriver and seven minutes with an electronic machine, he could “install a vote-stealing program” that would be hard to detect and shift a percentage of the votes.

In states like Pennsylvania, these voting machines “are delivered to polling places several days before the election — to elementary schools, churches and firehouses,” he said. That creates the risk of tampering. “This is not just one glitch in one manufacturer’s machine. It’s the very nature of computers,” he told a House subcommittee last month.