In 2000 it was hanging chad in Florida; in 2004 it was disputed provisional ballots in Ohio. Possible polling problems this year:

Two thirds of voters will use a different method to vote than they did in 2004. "Everyone's trying to figure out what works best," says Doug Chapin, director of Electionline.org. "That change breeds a lot of uncertainty."

Most voters will use paper because of security and accuracy worries about digital voting. Paper balloting provides a physical record, but it "lends itself to people making mistakes," like stray marks, says Paul DeGregorio, formerly of the Election Assistance Commission.

Voter turnout will be high. Primary issues could reappear: long lines, inadequate numbers of voting machines and ballot shortages.