Australian prime minister Julia Gillard (Penny Bradfield -- Associated Press)

Even though there's social pressure to vote in the United States, no one makes you go to the polls.

Other countries, however, take a heavier hand. According to the CIA World Factbook, 23 countries have compulsory voting, including Argentina, Brazil, Greece, Peru and Australia.

Some nations, such as Italy or the Netherlands, have mandatory voting but don't enforce it. And even in countries that do enforce it, the penalties are typically modest. Australia's punishments start with a AUS$20 (about $20.87 in the United States) fine, and scale up from there for repeat non-voters.

But studies suggest that even that amount for a fine changes the electorate considerably. One study found that in the 1996 election in Australia, compulsory voting reduced the vote share of the victorious right-wing Liberal Party/National Party coalition by five points. Another study looked at the interwar period (1918-1941), when Australia first embraced compulsory voting, and found that the policy benefited the left-wing Labor Party by about seven to nine points.

So does this mean that compulsory voting would help Democrats if adopted in the United States? Maybe. For one thing, poor Americans are less likely to vote than rich ones, but when they do vote, they tend to vote Democratic. If turnout rates were equal across income groups in 2008, Obama would have gained about 2.3 points, and McCain would have lost three, compared with their actual results.

But then again, it could be that poor non-voters tend to be more conservative than poor voters, so equal turnout could end up a wash. And, indeed, there's some evidence for the idea that compulsory voting wouldn't change things in United States. That's the argument of John Sides, Erick Schickler and Jack Citrin, who found that "nonvoters are just slightly more Democratic than voters," enough to tip a close election but not a game-changer. Some advocates of compulsory voting, such as Peter Orszag, the former White House budget chief, cite this as evidence for the proposal's lack of partisan bias.

For others, though, the change in results is the point. The American Enterprise Institute's Norm Ornstein and Brookings's Thomas Mann have proposed mandatory voting as a way to end the underrepresentation of poor peoples' say in Congress (they also want to give away the fines of non-voters to a randomly selected voter through a lottery, as an additional incentive).

And the evidence is mounting that such underrepresentation is a serious problem. Political scientist Martin Gilens at Princeton argues that policies benefiting the poor are far less likely to pass Congress than other legislation, even when as much as 80 percent of the public supports them.