By its nature, electronic voting is prone to both manmade and technical glitches.
"Voting machines are always going to have issues. That's inevitable," said Tova Wang of The Century Foundation think tank. "They're machines that are operated by human beings. The question is whether the poll workers are trained and have everything they need. If the machines malfunction, do they have paper ballots and do they have enough of them?"
Weather was a concern in some states. Snow or rain fell in states including Connecticut, Illinois, Kansas and Massachusetts, and elections officials worried that might discourage some voters. In Tennessee, where temperatures were in the 70s, a storm front threatened to bring hail and tornadoes.
Some Tennessee school districts, including Memphis, announced early closings so students could get home before the expected bad weather. Polls were still scheduled to remain open statewide until 8 p.m. EST.
"I would encourage voters, if they can go ahead and vote -- vote," said state Election Coordinator Brook Thompson. He urged local election officials to keep up with weather forecasts and conditions in their counties.
In Arizona, where voting activists feared a controversial photo ID rule could cause confusion, things were apparently fine. "People are walking up to the polls with their drivers' licenses in their hands," said Mindy Moretti, who was monitoring voting in the Phoenix and Scottsdale areas for the watchdog group Electiononline.org. "People seem ready for it. No one seems to be upset."
In the lead-up to Super Tuesday, voting advocates worried that long lines, high turnout and record numbers of mail-in ballots in states such as California could drag out the counting process for days. Across the country, election officials have estimated that mail-in ballots may account for as much as 50 percent of the vote in some areas.