The obvious precedent that comes to mind is 1998, when Liberal and Labor parties in Tasmania combined to reduce the size of the house of assembly, so as to increase the quota for election with the intention of wiping out the Greens. It didn’t work: the Greens still won a seat, their vote dropped only slightly, and twelve years later they’re back to five seats and more than double their pre-1998 vote.

And the Victorian Liberal Party hasn’t even got the benefit of bipartisan agreement. Instead of joining with Labor to shaft the Greens, they’re trying to do it on their own, while the Greens have the benefit of a better than ever preference flow from the ALP. What on earth were they thinking?

Perhaps the Liberals have started to believe what Labor’s spin doctors have been telling them, that the Greens are only a factor in Labor seats. Perhaps they believe the News Ltd line that the Greens are a threat to western civilisation and so preferencing against them will be popular in the suburbs. Or perhaps (as I suggested last time something like this happened) they were just throwing a temper tantrum.

On the first possibility, the Liberals risk scoring an own goal. In reality, Greens strength is spread all around the inner suburbs; of the 24 seats with the strongest Green vote in 2006, a third are already Liberal-held (with a couple more likely to join them this time). Without Liberal preferences, the Greens will have to stop thinking about seats like Footscray, Preston and Williamstown; instead they will probably put more work into Caulfield, Hawthorn and Kew. That could give the Liberals some anxious moments in 2014.

As to the second motive, it may be true that the very mention of the Greens will make outer-suburban and country voters rally to the Liberal flag. This seems to be an article of faith among many on the Liberal right, but it’s at best an unproven thesis — the Tasmanian experience is certainly not encouraging. And many voters who feel that way about the Greens are probably none too keen on the Labor Party either, so they might not appreciate the idea of Labor cabinet ministers being rescued by Liberal preferences.

Which leaves us with the idea that the Liberals are just acting in a fit of pique. If so, this may not be such a bad time to do it. Unlike 2006, when Liberal preferences almost gifted Labor control of the legislative council, there is not much at stake here that the Liberals should really care about, so there’s something to be said for taking the opportunity to show the Greens that they can’t be taken for granted.

It’s not even clear that it’s such a big benefit to the ALP. Although the morning papers are repeating the line that Labor is now safe in the inner-city seats, I think a commenter on Antony Green’s blog last night was probably closer to the truth when they suggested that this makes Melbourne and Richmond more rather than less of a contest — with Liberal preferences against them, Labor could have just written those seats off, whereas now (as my colleague William Bowe explains) they will be line-ball.

It remains to be seen whether the Liberals’ decision will hurt the Green primary vote. It might, by reducing the aura of inevitability about their progress. But it seems more likely that it will work to their benefit, as the Greens portray themselves as the only independent voice standing out against the their colluding opponents in the “old parties”.

If the Liberals really thought they could wipe out the Greens, they’ve left it much too late. At some point the two will have to deal with each other, and having vented a bit of their aggression yesterday the Liberals might just find that easier in the future.