Mandatory voting would give a lot of power to the gerrymanderers. As we know, modeling the behavior of large groups of people is easy. When you can predict who will vote and you have access to drawing voter district boundaries, you have the ability to preselect who will be elected.

Not voting is a choice and allows voter apathy to play a role in deciding elections. If an area is a majority Democrat but most of the voters don't care enough to vote then a motivated minority party can put boots on the ground and get their candidate into office. With mandatory voting that would never happen as the majority incumbent would always win thanks to receiving all those votes cast out of obligation rather than intent. That aggravates the most difficult part of democracy which is giving people with unpopular yet valid opinions a voice in the conversation.

Now, if we had proportional representation...

Mandatory vote is a good start, and will encourage more people to think about their government. In addition, get rid of electoral college. Add verifiable secure online voting and promote a system wide education of the people on the benefits of political envolvement.

I'm curious what impact mandatory voting has on the number of viable political parties. In Brazil, we have mandatory voting and a few more viable political parties (at least at the state and local level). I haven't a clue if there is any relationship, but I would imagine that requiring people to vote would involve having the 63% that isn't voting currently looking at candidates from the two parties and saying to themselves "I hate both, who else is available?"

Well in Brazil we do have mandatory voting and our political situation is one of the worst in the world.. Everybody here is trying the opposite - to make voting not mandatory.

The population does not feel represented by the politicians, yet we have to choose one of them every election. Buying votes /manipulating electors in this scenario looks really easy..

It if must be done, it must be done with a radical reform of ballot access, low electoral ceilings and easy-to-register third parties. However, it's not a panacea. Greece, Argentina, Brazil managed to have quite corrupt, dirty politics even with mandatory voting.

I think switching to instant runoff voting would have more impact. That way, folks can vote their conscience first, practicality second.