Obama recently suggested that mandatory voting would really benefit the United States, here is the video where he states this:
Besides the moral issue that mandatory voting involves forcing people at the threat of imprisonment to pick between two candidates who, when elected, will continue to oppress them, mandatory voting also involves forcing the around 50% of the population that currently does not vote, to vote. You know what those people are? you probably know a few of them, they come in two varaties, apathetic, and stupid.
The apathetic ones don't have any opinions on politics, they cannot tell you what kind of government they would like to live with, they don't have any sort of political philosphy, and they really don't know any current issues with the government. They can be very nice people, and infact are much more desireable to be around than a died in the wool Republican or Democrat, but the fact that they will be forced to vote now means they have to pick from candidates they know nothing about. This would probably further marginalize third parties, because none of these people even know they exist, so voting for them will not even be an option they will consider. Plus, it will already contribute to the problem of people choosing candidates based on their charm and good looks, something charismatic Obama probably is hoping for.
The other group, the stupid, are your classic white trash, high school dropout, redneck, fuckwads. These r the people that discussing politcs with will make you shiver where you stand. These are the people that want to shoot all the mexicans, force every product to be made in America, and think the military is the greatest organization on Earth. Most of them are two illiterate and lazy to try voting at the moment, but when they are forced to do it, all bets are off, and I dont know about you, but if mandatory voting enters the United States, I think I'll be exiting it.