Here’s a quick graph that I threw together for a lecture I’m giving tomorrow, based on aggregates of the latest polls and seat predictions (all garnered from the excellent May2015 website). Click for a bigger version!

The back row of columns represents a reasonable guess of the real distribution of seats, based on the latest polls, within our current First Past the Post system. Specifically, it makes use of the May2015 Predicts totals, which is a good model, incorporating the ‘strong transition’ equation as well as data from Ashcroft’s polling of marginal seats.

The front row of columns, meanwhile, shows a seat distribution that assumes simply that the aggregate of current polling is accurate and that each party’s nation-wide support level is translated directly into proportional Westminster seats. This would be the outcome under some variant of a ‘list’ electoral system, and would be close to the outcome under STV (depending on how that system would be implemented).

There’s lots to talk about in this graph, just as there is for so many comparable images based on the outcomes of past elections. Regionally-bound parties like the SNP would clearly have less nation-wide power under a proportional system. Parties with meaningful levels of support that are quite widely distributed would do better. How resilient the Lib Dem MPs would be under a more proportional system! How significant UKIP would be for everyone’s post-election coalition mathematics! How much more seriously we’d have to take the Greens!

But the real headline – and the thing that is made most immediately and obviously visible by illustrations such as this one – is the changing size of those first two red and blue columns. The two parties that will necessarily be involved in every future government are also the two parties with the most to lose from a more proportional voting system.

This doesn’t add up to a substantive argument about the value of proportionality of representation. But I don’t think we need to look any further if what we want is an explanation of the failure to make meaningful reforms to FPTP, and the gloomy prospects for such reform efforts in the future.