Some 66 million voters, or about 38 percent nationwide, will be able to use e-voting machines in Tuesday's elections, according to Election Data Services Inc., a Washington-based consulting firm.

Matt Zimmerman, a staff attorney for the San Francisco-based Electronic Frontier Foundation (EFF), a nonprofit privacy and digital rights group, said that the states being closely watched for potential problems include Ohio, Florida, Tennessee and Pennsylvania.

"There's certain places [being watched] like Ohio and Florida -- just because they're Ohio and Florida and they're going through procedural problems that we've seen" in the past, Zimmerman said. "Over the last few election cycles, these states have had issues."

In Tennessee, many counties have bought new electronic voting machines that will be used for the first time, leading to concerns about potential problems with the rollouts, he said. In Pennsylvania, there are concerns about whether e-voting machine vendors made needed changes to their equipment -- and whether the machines will work as expected, Zimmerman said.

"Using past elections as a guide is tough because so many more jurisdictions are using these machines for the first time," he said.