"That (comparison) process with the computer terminals is very slow, and that can create some long lines," said Clare Schexnyder of Election Protection, a national election monitoring group. "We're finally figuring out that it's not that there are not enough voting machines, it's the check-in process."

By its nature, electronic voting is prone to both manmade and technical glitches. In the South, voting was affected by acts of nature.

"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.

Tornadoes also hit Arkansas. Natasha Naragon, a spokeswoman for the secretary of state's office, said several polling locations closed in northern Conway County.

"It's been a wild night," state emergency management spokesman Tommy Jackson told Little Rock television station KATV. "A heck of a way to have elections in Arkansas."

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 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.

More than 5 million people have requested mail-in ballots in California, where there are 15.7 million registered voters. Election officials in the most populated and delegate-rich state in the country have said results may not be available until Wednesday or later.

As much as 25 percent of the overall vote may go uncounted Tuesday night, officials said.