If we have a look back in the polling rear view mirror, one of the things that stand out is the consistency of the gap between each of the major party primary vote estimates and their respective two party preferred estimates. To highlight, we’ll run a couple of charts using all of the polls taken by every pollster since January 2008.

What also stands out is that the Labor “vote gap” is much larger than the Coalition’s, reflecting the beneficial preference flow that Labor receives from the non-major party vote (read Greens).

If we compare the primary/two party “vote gap” for each of the majors since January 2008, we get something that looks like this (I’ve used a locally weighted polynomial regression as a trend line to knock out the polling noise).

Since October last year, the 10 point differential between Labor’s primary and two party preferred estimates has been rock solid, as has the Coalitions six point gap. In fact, during the course of the whole Rudd Government, the gap has only moved within a two point range for each party on trend estimates. This suggests to us that the Liberal Party is experiencing a structural two party preferred deficit to Labor of around four points.

This has long been a problem for the Coalition — Labor has hoovered up a larger share of minor party preferences ever since One Nation left the scene. As long as the Coalition ran a primary vote four points higher than Labor — as they tended to do under Howard post-1998 — they were pretty much safely ensconced in government.

But the Australian political landscape has significantly shifted since then. As George Megalogenis has pointed out a number of times, the combined primary vote of the broad centre-left in Australia has formed a clear public majority. We can see this by summing the Newspoll ALP and Green primary votes and running a trend line through them (here we’ll use just Newspoll figures):

This brings us back to the Coalition primary vote deficit problem. Under the last few elections of the Howard government, they needed to be on a primary vote around four points clear of Labor’s, but if we look at the primary vote differential between Labor and the Coalition as an All Pollster trend line over the term of the Rudd government, the enormity of the Coalition’s problem here becomes stark:

For the Coalition to be returned to government, that Primary Vote Gap needs to be a negative four rather than the double digit positive numbers it’s been in for most of the last 16 months.

Even though the Coalition has pulled a couple of points from the ALP primary and added it to their own since April, that is simply small change compared to what is required. A 54/46 or even a 53/47 poll result isn’t the manna from heaven that Coalition backbenchers are being lead to believe — sure it’s not another 58/42, but in the broader scheme of the Coalition pursuit of the Treasury benches, it’s barely even a start.