Anna Bligh’s thought bubble, in which the Queensland premier on Tuesday floated the idea of returning the state to compulsory preferential voting, has not gone down very well. Today’s Brisbane Times presents figures that undermine the case for change, and quotes opposition leader John-Paul Langbroek saying that “Queensland Labor wants to cheat its way to the next election”.

Possum Comitatus had all the graphs yesterday so you can judge for yourself whether compulsory preferences at state level would make much difference to federal informal voting. There’s no doubt it would increase informal voting at state elections, which Bligh seems unconcerned about. But either way, it’s pretty obvious this is primarily about politics, not about the informals.

And to understand the politics of the debate, it’s necessary to appreciate the extent to which political parties are captive to their histories. Preferential voting was introduced — in 1918 federally, and at varying dates from 1892 (Queensland) to 1929 (South Australia) — by non-Labor governments to prevent their votes from being wasted when they had multiple candidates. For most of its history, it clearly worked in their favor; first the Country Party and later the DLP could contest seats against the main non-Labor party, and their votes would still be counted as preferences.

For the same reason, Labor generally opposed the system and dreamt of going back to first-past-the-post. In Queensland they actually succeeded in doing so in 1942, but preferences were reintroduced by a Coalition government in 1962. After that Labor pretty much resigned itself to preferential voting, but aimed to at least make preferences optional, knowing that this would result in votes leaking away from the Coalition in three-cornered contests.

Optional preferential voting, however, is not just a compromise between compulsory preferences and first-past-the-post; it makes sense in its own right because it respects the will of the majority of voters without forcing people to express preferences if they don’t want to. (It is the logical counterpart to optional voting in general, although of course that debate as well is usually driven by partisan considerations.) So Labor state governments generally enjoyed popular support to introduce optional preferences in New South Wales in 1980 and Queensland in 1992.

In the meantime, however, the political dynamics were shifting. In the 1980 federal election, for the first time, Labor gained more out of the preferential system than it lost. The DLP had disappeared, the Coalition parties rarely ran against each other, and Democrat preferences were tilting more towards Labor.

That trend has continued and become stronger with the rise of the Greens. At this year’s federal election, for example, if preferences had not been counted the Coalition would have won comfortably. Labor won nine seats — Banks, Corangamite, Deakin, La Trobe, Lilley, Lingiari, Moreton, Reid and Robertson — after trailing on primary votes; the Coalition won none (although other candidates came from behind in Denison, Melbourne and O’Connor).

But over most of the last 30 years, the non-Labor parties have remained hostile to optional preferential voting: partly because it puts a loaded gun in the hands of a recalcitrant Coalition partner, but also just through historical habit. Generations of Liberals have been taught that optional preferences are a dastardly socialist plot, and old habits die hard.

If Queensland is any sign, generational change finally seems to be having an effect. The opposition — now united in one party, so with no worries about Coalition issues — is defending optional preferences, knowing they hurt Labor by allowing Green preferences to just leak away (and at an increasing rate as more voters get used to the system). Bligh, for the same reason, would love to return to compulsion.

But it’s hard to think why voters should be forced to give preferences to parties that in fact they are indifferent between. No one denies that it’s inconvenient to have different systems at state and federal level, but the answer to that is to make preferences optional in federal elections as well — not to deprive state voters of a legitimate democratic choice.