(CNN) - Eight years after Florida's hanging chads exasperated voters and helped usher in sweeping changes in voting technology, many election observers remain concerned about the accuracy of the electronic voting systems most Americans will use November 4.
Touch-screen machines can occasionally fail or register votes for unintended candidates. Optical-scan systems can have trouble reading paper ballots that are too long or marked with the wrong ink. At least one study suggests that electronic voting machines can be easily hacked.
And some 9 million voters, including many in the battleground states of Ohio and Florida, will use equipment that has changed since March, increasing concerns about errors next Tuesday.
"You can be almost certain that there will be irregularities in some places around the country," said Rep. Rush Holt, D-New Jersey. "The problem now is that roughly a third of voters nationwide will use unverifiable electronic machines. So if there are uncertainties, there will be no way to resolve them."
With early voting under way in 31 states, these problems have already surfaced. In recent weeks, voters in West Virginia, Colorado, Tennessee and Texas have reported that touch-screen machines registered their votes, at least initially, for the wrong candidate or party.