Some voting experts and campaign aides predict a record turnout of some 130 million voters, which would be the highest percentage turnout in a century, and would shatter the previous record of 123.5 million people in 2004.
"The level of voter enthusiasm and excitement indicates that this is a 100-year storm of elections," said Paul Gronke, director of the Early Voting Information Center at Reed College in Portland, Ore.
In Chesapeake Virginia, rain dripping off wet voters soaked some ballots, making them unreadable for the optical scanners. State Board of Elections Spokesman Ryan Enright told CNN that the polling station is collecting them in a secure container next to the polling machine and will tabulate them when they dry off. Once dried, these ballots will be tabulated by officials, Enright said.
Machines were reportedly failing in precincts from New Jersey to California, according to the Associated Press.
Voters in Texas, Missouri, Mississippi, Alabama and Arkansas reported receiving text messages telling Democrats to vote on Wednesday, the day after the election. An e-mail circulated in Arkansas also told voters they could still cast their ballots on Wednesday, according to CNN.
In Kansas City, Missouri, there were lines of over three hours because precincts had received the wrong voter registration lists, Jessie Sargent told CNN.
"There have been quite a few people leaving because of work and school, and they have no provisional ballots or books to sign in to," she said.
Election Protection, a nationwide, nonpartisan voter protection coalition, reported problems at some polling places resulting in long lines, some lasting two hours, causing some voters to leave without casting a ballot.
Election Protection's information comes from complaints made to its hot line, 1-866-OUR-VOTE.
Sen. Barack Obama, D-Ill., was one of the first to vote at his precinct in Chicago, accompanied by his wife Michelle and their two daughters. He planned a quick campaign stop in Indiana on Election Day before a massive outdoor rally in Chicago.
Sen. John McCain, R-Ariz., meanwhile, planned events in Colorado and New Mexico, then a party at the Biltmore Hotel in Phoenix.
The first town to report election results, tiny Dixville Notch, N.H., awarded Obama 15 votes to Mr. McCain's six. It traditionally votes Republican, and President Bush won the vote there in 2004.
Noting the long lines and unprecedented early voting numbers, party activists said the invigorated electorate will shatter records Tuesday.
"I haven't seen anything like this since John F. Kennedy," said Victor N. Farley, a former Erie County Republican chairman, reported the Buffalo News. "It's hard to think of any other people who have evoked such emotions."
"People who normally are not interested in this stuff stop me and ask, 'What do you think?'" said former Erie County Democratic Chairman Joseph F. Crangle, according to the Buffalo News.
In 2004 presidential election, 64 percent of voting-age citizens voted, up from 60 percent in 2000, according the U.S. Census Bureau.
Campaign staffers for both presidential candidates said they were expecting good results but also cited complaints about voting. Republicans are concerned about fraudulent voter registrations, while Democrats say eligible voters may not get a chance to vote because of ballot shortages or delays due to long lines.
"Ideally, everyone will be able to vote once and vote fairly," said McCain spokesman Ben Porritt, quoted the Washington Post.
"We're feeling good that [today] Americans who want to vote will be able to do so without a hassle," said Jenny Backus, a strategist with Obama's legal team, according to the Post.
Also up for grabs: all 435 House seats, along with 35 out of 100 Senate seats. In addition, voters in 11 states are electing governors.
Democrats currently control the Senate 51-49 but are hoping to get a filibuster-proof majority of 60. They also control the House 235-199, with Republicans looking to close the gap.
Several key races have grabbed national attention, including an apparent toss-up in North Carolina, where incumbent Republican Sen. Elizabeth Dole faces a challenge from newcomer Democrat Kay Hagan.
In Alaska, Democrat Mark Begich is running against Republican incumbent Sen. Ted Stevens, who was recently convicted on federal corruption charges but remains on the ballot.
And in Minnesota, Republican incumbent Norm Coleman's Senate seat is challenged by former "Saturday Night Live" writer, comedian and radio host Al Franken. Observers say the race could go either way.
Initiatives dealing with gay marriage, stem cell research and abortion also are on the ballot in several states.
California's Proposition 8 would terminate marriage rights for gay couples. Arizona's Proposition 102 would amend the state's constitution to define marriage as between one man and one woman. Similarly, Florida's Proposition 2 would amend the state constitution to ban gay marriage.
In South Dakota, a ballot initiative would restrict abortions statewide, except in cases of rape, incest or to protect the woman's health.
And in Michigan, voters will weigh in on two proposals: to allow people to donate leftover embryos produced in fertility clinics for research purposes, and to legalize marijuana for medical purposes.