Washington - Tens of thousands of Americans have already cast their votes for Hillary Clinton, Donald Trump or other candidates well ahead of November's presidential election, part of an early voting tradition that is gaining popularity.With 35 days still to go, it has not yet been a voting deluge. According to professor Michael McDonald, an early voting specialist at the University of Florida, about 130 000 people have already voted -- out of 130 million expected voters.Early ballots will be officially counted on Election Day, Tuesday November 8.The US electoral system is a decentralised operation: the nation's 50 states organise the vote and the ballot counting, each in its own way.Americans have two early voting options:1. By mail. This is available across the 50 states. Voters usually must request the ballots in advance. In 20 states, they must provide an excuse for voting by mail. Three states -- Colorado, Oregon and Washington -- mail ballots to every registered voter.2. In person. Voting booths, for example in county administration offices, are open early with varying degrees of access, in 37 states and the District of Columbia, according to the National Conference of State Legislatures.Voting in person largely begins in October. Three states have already begun in-person early voting, including Iowa. Ohio residents can begin voting in person on October 12. President Barack Obama is scheduled to visit Ohio on October 14 for a "Get Out the Early Vote" rally for Clinton in Cleveland, according to the Clinton campaign.There is no access to the ballots cast, but there are some clues about whom the early voters might be supporting.Americans register according to party - either as Democrats, Republicans, independents, or members of other parties - and electoral authorities sometimes provide statistics on the number of voters in each party who requested ballots. In Iowa, for example, twice as many Democrats as Republicans have requested absentee ballots, suggesting an advantage for Clinton.But such data cannot foretell an election result. The Democratic Party may have begun its grass roots mobilization earlier than the Republicans in Iowa. The elderly usually vote before young people, which favors the Republicans.Early voting is expanding. In 1996, according to the US Census Bureau, only 10.5% of votes were cast early. By 2012, that figure had risen to one third of all ballots cast.