(CNN) -- The unexpected wave of millions of early voters casting ballots for Sens. John McCain and Barack Obama may prompt Congress to mandate some form of early voting nationwide for future elections, experts say.
Early voters examine a sample ballot Friday in Charlotte, North Carolina.
1 of 3 more photos » So far, more than 24.4 million voters have cast ballots in states where early voting is allowed, providing convenience for voters and, in theory, rescuing poll workers from an overwhelming turnout on Tuesday.
This historic election is expected to fuel congressional support for a law that allows voters to cast early ballots without providing an excuse, said the director of the nonpartisan Early Voting Institute, professor Paul Gronke.
In many states that require a reason for early voting, mail-in voters must provide election officials with a formal excuse explaining why they cannot vote on Election Day. They often must obtain signatures from notaries or physicians before they can be granted a ballot.
"It is almost certain that after the election there will be legislation proposed in the next session of Congress that will mandate no-excuse absentee balloting nationwide," said Gronke. "This will especially be the case if Democrats take the White House." See where Americans are voting »
CNN Voter Hotline
If you have a problem voting or see a problem, call the CNN Voter Hotline at 1-877-GOCNN08 (1-877-462-6608); CNN will report on some of your calls, and our partner InfoVoter Technologies can help get you in touch with your election board or find your voting location. As of Monday, more registered Democrats than Republicans had cast ballots in the 26 early voting states where election statistics are available. Of 7.5 million voters who are registered by party in nine reporting states, Democratic voters outnumber GOP voters 57.3 percent to 42.7 percent.
Because these are statistics and not results, it's unknown which candidates the voters supported. Voters who register with political parties don't always vote for their party's candidates.
Early voting continued Monday in the critical battlegrounds of Indiana and Ohio. At a polling place in a heavily Democratic area of Franklin County, Ohio, lines were already long at 7:30 a.m. A voter at the head of the line said he had arrived at 4:45 a.m.
"For the most part, people are more excited than frustrated," reported CNN's Mary Snow. Watch final early voters in Ohio »
Early voting is already poised for expansion. On Tuesday, Maryland voters will decide whether that state should also offer early voting. Top GOP and Democratic party officials in Alabama told The Anniston Star newspaper they plan to present a broad early-voting proposal to the state Legislature.
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If voters and election officials in states without early voting such as Pennsylvania or Virginia are overwhelmed Tuesday by high voter turnout, it could accelerate political momentum for a national early-voting law, said professor Michael McDonald, who tracks early voting at George Mason University.
"States that have early voting may be better able to handle large numbers of Election Day voters because so many voters in those states have already cast ballots," said McDonald. "And that may be what will prompt people to re-evaluate the idea of some federal legislation mandating it nationwide." See photos of iReporters getting a jump start on voting »
Twenty-eight states now allow voters to cast absentee, mail-in ballots without providing an excuse, and in California, voters can automatically have their absentee ballots sent to their homes for every election. The convenience of early voting -- either by mail or in person -- allows voters who can't spend hours waiting to vote on Election Day to participate in the democratic process. iReport.com: Early voting teens spill the beans
Rep. Susan Davis, a California Democrat, sponsored unsuccessful House legislation in the past session of Congress that would force all states to offer voting by no-excuse mail-in ballot.
"It offered everyone a chance to choose the time that they can vote -- where they wouldn't have to worry that they will have to leave a child or loved one or miss work -- whatever keeps people from voting within certain hours on Tuesdays," Davis said.
Ballots so far: 24,498,379
States reporting: 26
Voters registered by party: 7,506,707 in nine reporting states
Of those: Democrats, 57.3 percent, GOP, 42.7 percent
Source: AP for the National Election Pool Federal legislation comes with political hurdles, Davis said, because "people do see this as a state issue. But the success of early voting in this election, and the models that are out there, might provide us with some of the details that we could put into a national bill."
New York Democratic Rep. Steve Israel is sponsoring a bill that would designate elections as a two-day weekend event in November.
"It's crazy to me that we're only allowing people to vote over a certain period of hours on one day," Israel said. "Other nations that have much higher voter turnout allow their people to vote over several days."
Obama's campaign has taken advantage of early voting as an opportunity to motivate new voters to register and get to the polls before Tuesday. Networks made up of thousands of Obama supporters have been working for months at the neighborhood level to spread the word about when early voting begins and where voters should show up to cast their ballots.
The excitement generated by this presidential race has shattered the expectation that early voters tend to be white and older and wealthier, said Gronke. In North Carolina and Georgia combined, more than a million African-Americans have cast early votes in recent days, according to election officials.
But before early voting goes nationwide, Gronke is calling for a national debate about its merits.
"We should first ask the question: What does Election Day mean?" he said.
According to one skeptic, early voting represents an important philosophical turn in the way the nation chooses its leaders.
Harvard University political philosophy professor Dennis Thompson said early voting would prompt candidates to "run two different campaigns, one directed toward loyal partisans and relatively well-off voters, the other tailored for less partisan and relatively less well-off voters."
Voting over a long period of time -- rather than only on Election Day -- also threatens to divide society, Thompson said, because early voters and their later counterparts make their choices based on different information.
After an early voter casts a ballot -- but before an Election Day voter does -- any number of events could occur that might affect a voter's decision. A candidate might pull out of the race, or be convicted of a crime, as was the case with longtime Alaska Sen. Ted Stevens.
Shifting voter opinion polls might prompt some voters who would otherwise have voted to remain at home on Election Day, said Thompson.
"But we shouldn't let this election determine how we think about future election reform," said Thompson, because it's not typical.
Early voting laws have given voters a new venue to express their enthusiasm. In Cleveland, Tennessee, scores of women -- some with their daughters in tow -- marched to the local courthouse to cast early ballots together in support of GOP nominee McCain. In Daytona Beach, Florida, administrators at Bethune-Cookman University shut down classes for a day to accommodate about 1,000 students who marched from the campus to the polls to vote early. See iReporters reveal who they're voting for and why »
The popularity of early voting has increased consistently since 2000, when early votes represented 16 percent of the ballots cast. The percentage increased to 22 percent in 2004. This year, experts predict as much as 33 percent of the nation's electorate will have voted early.
But massive turnout has also put a strain on election officials -- forcing some counties to extend voting hours to make up for voting machine glitches or to accommodate thousands of unexpected voters, some of whom were forced to wait in line for three or more hours before they could cast their votes.
Despite problems, McDonald sees a bright future for early voting.
"There aren't that many states left that don't use it," he said. "So, will this election prompt passage of some sort of federal early-voting legislation? I think there's a good chance."