General Discussion

Australia has had mandatory voting for eligible voters (18+) for a long time. It works like this:

Prior to elections, the Australian Electoral Commission updates the electoral roll of all eligible voters. On election day, voters have their names crossed off the roll at whichever polling place they attend.

After the election, the electoral roll is cross-checked against voter records. Anyone who didn't vote and can't provide a valid reason (for example - illness, living remotely, religious beliefs) is issued a $20 fine by the AEC. If not paid, this can escalate to further fines of around $180 plus court costs if convicted.

Over 180,000 penalty notices were issued after the 2022 federal election to enforce the compulsory voting laws. While controversial to some, the system has maintained over 90% voter turnout in Australia for nearly a century.

A similar system would probably moderate political extremes in the US. I think any fine that is used as a means of enforcement needs to be scaled to the means of the individual being fined in order to not disproportionately target lower wealth individuals (but an elimination of the enforcement fine completely for the lower end of the wealth scale would maybe ironically result in less from that group voting and thus give them disproportionately lower representation in outcomes).