Since e-voting machines may not even indicate that a problem has occurred, there's no way to ensure that a voter's intentions are carried out, Bhatia said. "We need a safety net in case of fraud or machine failure," she added. "A routine manual audit and voter-verified paper records provide that safety net."

A voter-verified paper trail is a printout that the voter can see before he has his votes tallied. That paper trail includes the names of the candidates voted for, and that paper is then saved as part of the voting record in case it is needed for recounts or reviews.

"The voter-verifiable step in the process is the crucial point," Bhatia said. "It's the check and balance."

Harry Van Sickle, the commissioner of elections for Pennsylvania's 67 counties, said county elections officials across the state "have a quiet confidence" about how the election will go Tuesday using e-voting machines. "In talking with most of the election directors, we find out they feel good about [the equipment]," he said. Fifty-four of the counties are using touch-screen machines, while 13 use optical scanners.

"We are basically ready for the election," Van Sickle said Monday. "It seems to be going pretty well."