Republicans would hate Compulsory voting in the United States, the 99 percent heading to the polls! If you don't attend on voting day, you are heavily fined! In South America the Witnesses will leave their ballot blank or check nobody.
From Wiki
" Compulsory voting may encourage voters to research the candidates' political positions more thoroughly. Since they are voting anyway they may take more of an interest into the nature of the politicians they may vote for, rather than simply opting out. This means candidates need to appeal to a more general audience, rather than a small section of the community.
A result of this setup is that it is therefore more difficult for extremist or special interest groups to vote themselves into power or to influence mainstream candidates. Under a non-compulsory voting system, if fewer people vote then it is easier for smaller sectional interests and lobby groups to motivate a small section of the people to the polls and thereby control the outcome of the political process. The outcome of a election where voting is compulsory reflects more of the will of the people (Who do I want to lead the country?) rather than reflecting who was more able to convince people to take time out of their day to cast a vote (Do I even want to vote today?).
Wiki "Enforced
These are the 10 countries that enforce compulsory voting:
Argentina - Compulsory for citizens between 18 and 70 years old, non-compulsory for those older than 70. (However in primaries, citizens under 70 may refuse to vote, if they formally express their decision to the electoral authorities, at least 48 hours before the election. This is valid only for the subsequent primary, and needs to be repeated every time the voter wishes not to participate.)
Australia - Compulsory enrollment and voting for state and national elections for all eligible adults (18 and above). In some states local council elections are compulsory too.[10]
Brazil[11] - Compulsory for citizens between 18 and 70 years old.
Democratic Republic of the Congo
Ecuador - Compulsory for citizens between 18 and 65 years old; non-compulsory for citizens aged 16-18, illiterate people, and those older than 65.
Luxembourg - Only for the regionals and if signed up
Nauru
Peru - Compulsory for citizens between 18 and 70 years old, non-compulsory for those older than 70
Singapore[12] - Compulsory for citizens above 21 years old on the 1st of January of the year of election
Uruguay
There is one region in Switzerland that enforces compulsory voting: