With the rumour mills and gossip factories working overtime on just which Coalition pollies will exit Parliament and when, it might be time to take a quick squiz at the four most popularly mentioned by-election possibilities over the near term, particularly in light of the dismal polling that’s turning up all too regularly for the Coalition of late.

The four seats we’ll look at today are Costello’s seat of Higgins, Downer’s seat of Mayo, McGauran’s seat of Gippsland and Mark Vaile’s seat of Lyne.

If we look at the two party preferred margins of these four seats over the last 7 elections going back to 1990, it gives us an idea of the longer term demographic forces at work in these electorates.

The first thing that stands out here is the slow decline in the size of the margin that the Coalition enjoys in Gippsland, where the 2PP vote has been reduced from the 68.9% it achieved at the 1990 election down to the 55.9% it currently sits on. From these four seats, Gippsland is probably the greatest chance to change hands at a by-election, particularly if it becomes a three cornered contest. Continuing demographic change combined with the honeymoon of the Rudd government and poor Coalition polling makes this seat an interesting prospect from Labor’s perspective, especially since its current margin of 5.9% is well within the ballpark of the swings we seem to be seeing from the large pollsters.

If we graph the swing that each of these four seats have experienced over the same timeframe, where a positive swing is a swing to the Coalition and a negative swing is a swing to Labor, we also pull out a few interesting things. We’ll also throw in the national swing to give us an idea on how these individual seats behave compared to the national average.

What’s disturbing for the National Party here is the result in Lyne. Over the last 4 elections, the swing in Lyne has nearly perfectly mirrored the broader national swing. With every poll showing Labor support jumping between 5 and 10 points from their 2007 election result, Lyne becomes a cause for concern, sitting on a margin of 8.6%.

But the real problem for the Nats in Lyne is the possibility of a 3 cornered contest, as the last 3 cornered contest in Lyne back in 1993 had Mark Vaile coming in over top of the Liberal candidate by a mere 233 votes to take the seat.

The other big problem for the Nats in Lyne is Rob Oakeshott, the independent Member for Port Macquarie in the NSW State Parliament. If he threw his hat in the ring, the seat would simply be his. He would dominate the vote in the northern half of the seat (his current State electorate) which would carry him above the Nats and the Libs, picking up preferences along the way to knock out the ALP if they made it through to the final round of preference allocations.

Mayo and Higgins are different kettles of fish altogether, while we can see that the Coalition vote in Higgins is slowly eroding over time, for Higgins and Mayo to turn politically red would be the upset of the decade – it would take a serious bout of leadership failure for these seats to turn feral.

But what is certain is that Nelson’s poor leadership performance in the polling suggests that he is not only failing to bridge the disparate views that make up the twin support bases of the Coalition (with their affluent metro seats like Higgins and Mayo on the one hand, and their regional seats like Gippsland and Lyne on the other), but that Nelson has successfully alienated both groups. At 37% two party preferred, that drop in support isn’t coming from a single place, it’s bleeding from everywhere.

Any string of by-elections for Nelson would bring the problems of the twin support bases of the Coalition to front and centre, as well as his thus far demonstrated inability to manage their opposing interests in a way that doesn’t destroy his own political standing in the electorate and the Coalitions vote share as a consequence. Poor leadership here could endanger seats which shouldn’t even be on the map — especially in by-elections where the retiring sitting member generally takes around 1.5% of the party vote with them when they go.