[amaterasu]

Not meaning to be ignorant here, but how?

[gizmo686]

Compelled speech.

You could probably write the law in a way that avoids first amendment implications. For instance, require that people show up at a polling place, but do not require them to submit a ballot.

[pitaj]

Voting is political speech. The right to say nothing is as important as the right to speak.

[Spooks]

Have an empty spot you can vote for. So they still have to go through the steps and send it in, but they don't actually have to speak/vote on something

[ben-schaaf]

You have every right to leave the ballot blank.

[jagger27]

Or write Big Bird in.

[alextheparrot]

Voting is a form of speech, so you can’t be compelled to speak or not under threat of punishment.