The one obvious effect of compulsory voting (or, to satisfy the
purists, “compulsory attendance at polling booths”) is that it
increases voter turnout. Although compulsory voting is a rare beast,
all democracies have some experience of the effects of higher and lower
turnout, so we can be reasonably confident about its effects. When
turnout falls, those who stop voting are predominantly (a) the young
rather than the old, and (b) the poor rather than the middle class.

Conventional wisdom says that these two effects favour the parties of
the right rather than the left, and a move to optional voting in
Australia can be expected to favour the Coalition. It’s beyond doubt
that parties of the right over time have made it harder for people to
vote and parties of the left have made it easier.

that expectation that voluntary voting would favour the right might work against the government: voters do not take
kindly to politicians who are seen to be manipulating the system for
their own advantage. The backlash against such a move could easily
outweigh any gain.

In any case, the advantage for the right is by no means certain. Even
if the people who stop voting come disproportionately from the working
class, it doesn’t follow that they will necessarily be that part of the
working class that votes Labor. Furthermore, Howard as prime minister
has identified himself so strongly with the idea of appealing to the
workers (“Howard’s battlers”) that it is hard to see him embarking on a
political strategy that involves admitting that appeal is just a myth.

Similar points can be made about younger voters: it is an important
part of modern right-wing mythology that younger voters, especially
“Generation X”, are more conservative than their parents and vote

One further point has been suggested to me: if compulsory voting really
advantages the left, you would expect the ALP to have been more
successful than its counterparts in other western democracies, almost
all of which have optional voting. But if anything, the reverse is
true: Labor has been out of office federally for all but 16 years since
1949. Perhaps with optional voting they would have been even worse off, but the case is far from proven.