The US presidential campaign today enters its final week. But for many Americans, the election is already over: they have taken advantage of provisions that allow votes to be cast ahead of polling day, either in person or by post.

Because US elections are governed by state law, the rules vary from state to state, but many states now provide for either “early voting” — polling stations being open for about two weeks before polling day, which in Australia we call “pre-poll voting” — or postal voting, which the US confusingly calls “absentee voting” (a term that refers to something different in Australia).

Observers have been struck by the huge numbers turning out for early voting, particularly in some battleground states. Sean Quinn at fivethirtyeight.com visited an early voting centre in Charlotte, North Carolina, at the weekend: “We tracked movement and extrapolated the wait to be roughly an hour and a half. Nobody in line left, and nobody appeared anxious to leave.”

Most of the enthusiasm in early voting seems to be pro-Democrat. The New York Times last week reported that “Significantly more Democrats than Republicans have cast ballots” in such key states as North Carolina, New Mexico and Ohio.

It’s easy to get confused about the political effect of early voting. In past years, we know early votes have tended to favor Republicans over Democrats. But it doesn’t follow that the availability or increased use of early voting benefits the Republicans: if it just means the same people are voting at a different time, there should be no net political effect.

In Australia, for example, postal and pre-poll votes go more strongly to the Coalition parties than ordinary votes. There are also more of them than there used to be: up from 6.1% of the total in 1993 to 13.7% last year. But there’s no reason to think the Coalition has benefited from that trend.

Early voting is only likely to change the political outcome if it means an increase in the total turnout. In Australia, any such effect will be small because of compulsory voting. But in the US, this year looks like seeing a big increase in turnout, of which the crowds at early voting centres are a symptom.

There are other factors at work as well; problems in the past with crowded voting centres and malfunctioning voting machines have clearly led many voters to want to get in early as possible. But more liberal early voting rules are helping that happen, and it’s as close as you’ll get to a universal rule in politics that making it easier for people to vote benefits the parties of the left.

Quite apart from the increased turnout, early voting is something of an insurance policy for Barack Obama. People who make their decision now represent votes that can’t be changed later, so he’s protected against something dramatic happening in the last week to erode his lead in the polls. It also means he’ll get less advantage out of a late surge, but right now that’s probably the least of his worries.