If you look at the percentage of the electorate that turn out to vote how can you be sure that they were able to come and vote. Even if they choose to not vote it is harder to fake the results and less chance of electoral fraud. I feel that it would also make people think more about their vote.
In the UK generally it is the older generation 50+ who choose to vote, unfortunately they have a first past the post system so say 10000 people vote a, 10001 vote for b and 10002 vote for c then c will win even though 20001 people did not vote for them. This would also not be solved solely through a form proportional representation or collegiate system, 58.2% turnout at the 2012 us presidential elections so 41.8% of the voting elegible did not cast a vote/no vote.
I also think that it is not a good idea because then there are gonna be more rioting problems in the US, which will add up to immigration reforms & so forth. So no mandatory voting please or it will mess up our government of the United States of America.
Any time the government starts to make any particular political action mandatory, individual freedom is starting to be eroded. We should encourage people to vote in the United States, of course. But making someone go to a polling booth would be both impossible as well as unfortunate. It smacks of despotism.
If voting became mandatory for all citizens, it would no longer be a right, but a chore. If an individual believes that no candidate is worthy of his or her vote, and thus declines to provide one, then that should be his or her right. Voting is a choice, and should not be forced.