Early voting in the US presidential election will begin in two states tomorrow, seven weeks ahead of election day on 6 November.
Idaho and South Dakota are the first states to begin early voting, although North Carolina has been accepting absentee ballots by mail since 6 September.
By the end of September, 30 states will have begun either in-person or absentee voting, and eventually all the states will join in.
Much of the focus of the early voting period will be on the politically divided states of Ohio and Florida, which could be crucial in deciding the race between Democratic President Barack Obama and Republican Mitt Romney.
The states have also been the venues for court battles in which Democrats have accused Republican-led legislatures of trying to limit early voting periods in order to suppress turnout of working-class and minority voters.
Such voters make up large percentages of those casting ballots before election day and they tend to back Democrats. Absentee ballots, popular among military voters, tend to favour Republicans.
Restrictions on early voting are among several election laws passed by Republican-led legislatures since 2010.
Other new laws, also challenged by Democrats and voting-rights groups, have been aimed at limiting voter registration and requiring voters to show photo identification. Republicans have said the laws are aimed at preventing voter fraud.
The push to limit early voting came after Mr Obama's aggressive early voting campaign in 2008 helped propel him to victory over Republican Senator John McCain and showed the power of the bloc of early voters.
Early voters are particularly coveted because once they cast their ballots the candidates they support can turn their focus to attracting undecided voters.
Early voting accounted for a record 30% of all votes cast in the 2008 election. In Florida, more than half of the votes were cast before election day.
More than half of the African-Americans who cast ballots in Florida four years ago – 54% - voted early, nearly twice the rate of white people.
In five of the state's 67 counties, at least two-thirds of African-Americans cast their ballots before election day, according to a Reuters analysis of state voter data.
"It's pretty powerful evidence that African-Americans have come to rely on early voting," said Daniel Smith, a political science professor at the University of Florida.
"The change in early voting is not yet finalised in Florida," he added, noting that several lawsuits were pending on the new laws.
Since the 2008 election, Florida's Republican-controlled legislature has reduced the number of early voting days to eight from 14, a plan that drew criticism from Democrats and voting-rights groups.
In Jacksonville, Florida yesterday a federal judge heard arguments in a lawsuit filed by Democratic Representative Corrine Brown. She wants the court to overturn the new Florida law, which also eliminates voting on the Sunday before election day.
The law has already been the focus of a lawsuit by the US Justice Department, which under the Voting Rights Act must approve election changes in five Florida counties with a history of racial repression.
After a federal court said last month the new law unfairly hurt minorities, the state appeased the federal government by agreeing to extend the voting hours on each of the eight remaining days of early voting.
However, the new lawsuit filed by Ms Brown also takes issue with a part of the law that eliminates voting on the Sunday before the election, saying it unfairly affects African-Americans, a sizeable number of whom tend to vote after going to church.
Absentee voting in Florida begins on 2 October. In-person early voting is now allowed from 27 October through 3 November.
There also is legal wrangling over early voting in Ohio, where a federal judge recently overturned new early voting restrictions, saying the state did not have a strong enough reason to change the rules. The state government has appealed.
In 2011, Ohio legislators passed a law eliminating early in-person voting the last three days before election day for everyone but members of the US military. Supporters of the changes said they would help prevent voter fraud and give election officials more time to prepare for the election.
However, Mr Obama's campaign and Democrats sued to reinstate early voting for everyone in the last three days before the election. Democrats said about 93,000 Ohio voters used the last three days of early voting in 2008.
Even as Republicans have sought to restrict early voting, they are showing signs of being more effective in getting their supporters to cast ballots before Election Day, an indication that Mr Obama's success in 2008 made an impression.
During the bruising Republican primary battle, Mr Romney's focus on early voters in Florida, Ohio, Michigan and Wisconsin helped him win the primaries in those states.
Mr Obama's 2008 campaign perfected an aggressive strategy of targeting early voters and holding rallies in states once early voting had begun, to encourage supporters to vote right away instead of waiting for election day.
This week in Ohio, where in-person early voting begins on 2 October, Mr Obama reminded supporters to vote early. He targeted younger voters, who tend to favour him but are among the least reliable groups of voters when it comes to showing up at the polls.
"You can start showing up and voting on October 2nd - that's 15 days away," Mr Obama said on Monday in Columbus, drawing laughs when he added: "I see some young people here. Young people, you got to use early vote because you might not wake up in time on election day."
Michael McDonald, a political science professor at George Mason University, said early voters tended to be better informed partisans who know whom they wanted to vote for. The challenge, Mr McDonald said, was to make sure they cast a ballot.
"We have a specific strategy for each state to capitalise on early and absentee votes," said Rich Beeson, political director for Mr Romney's campaign. "We understand the importance of maximising those votes prior to election day."