When sitting down to dinner late last month, Tasmanian voters were given the clearest insight yet into the shape of their island’s Labor-Greens relations. In a remarkably blunt piece of campaigning ahead of the March 15 state election, Tasmanian Labor sent automated “robocalls” to the state’s voters, notifying them of why the party was breaking ranks with the parliamentary Greens.

The call, using the voice of ALP member Norm Britton, said:

I’m just calling to let you know I strongly favour Labor removing the Greens from government and ruling out governing with them again. Labor has sent a strong message that its priority is creating jobs and the best way to do this is for Labor to govern alone without the Greens.

When viewed alongside Tasmanian premier Lara Giddings’ sacking of Greens MPs from cabinet and her decision to ensure the legality of the Tamar Valley pulp mill (a project described by Greens leader Nick McKim as a “dinosaur”), it’s clear that Tasmanian Labor-Greens relations are as rocky as they have been since the minority government formed in 2010.

This strained relationship is not, of course, restricted to Tasmania. In February 2013, the inverse occurred. Federal Greens leader Christine Milne ended her parliamentary alliance with the ALP, citing Labor’s supposedly cosy link with “big miners”, a link allegedly leading to a diluted mining tax.

It’s no coincidence that these splits both occurred within sight of elections. In Tasmania, Labor is staring down the barrel of its first defeat since 1998. In February 2013, the federal Greens were facing a serious backslide from their breakthrough in 2010. For both parties, a close association with the other is considered an electoral liability.

But why is this the case? What forces erode the longevity of Labor-Greens minority governments, both federally and in Tasmania?

To answer this question is to acknowledge a number of contradictions within the modern political left, contradictions that make cross-party cohesion difficult.

Voter bases create divide

Firstly, there is the issue of social bases. Put simply, the ALP is forced to balance the preferences of a more diverse voting base than a Greens party with a relatively homogenous electorate.

The last time the ALP pieced together a successful electoral coalition (in 2007), it contained an even spread of electors with no post-school qualifications, non-degree qualifications and university degrees (winning 47%, 43% and 47% of each group).

Conversely, the Greens’ vote was concentrated overwhelmingly amongst the educated (with 6%, 6% and 16% respectively). The same applied to manual and non-manual professions (55% and 40% for the ALP, and 5% and 10% for the Greens) and inner-urban and suburban electorates, with Labor gaining seats in both areas, and Greens receiving support disproportionately from inner-city voters.

What this means in practice is that, on certain policy questions, the ALP’s electoral diversity cultivates a more cautious approach to reform. For the Greens, a party of educated social liberals, legalising something like same-sex marriage is obvious. For Labor, a party comprised of blue- and white-collar voters, alongside a considerable ethnic base, the issue requires the consideration of different groups and the weighing of their often conflicting preferences.

Attitudes to growth differ

Secondly, the relationship raises fundamental questions of economic and environmental philosophy. For Labor, a party whose political project has always demanded low unemployment and sufficient revenue for social services, economic growth is a self-evident priority.

For the Greens, a party whose social project involves post-material goals like environmental protection and the maximisation of “quality of life”, the growth paradigm is questioned. Their platform goes so far as to call it “incompatible with the planet’s finite resources”.

AAP/Rob Blakers

The co-existence of these priorities causes problems when the parties deliberate over policies that involve both considerable economic opportunities and dangerous environmental implications. In both cases that minority governments split, this clash was cited. For the federal Greens, it was Labor’s relationship with coal mining. For Tasmanian state Labor, it was the jobs associated with the Tamar Valley pulp mill.

Support groups clash

Finally, the interests of affiliated organisations amplify these tensions. Relevant trade unions tend to support the same controversial projects opposed by environmentalists. To protect their members’ jobs, the Australian Workers Union backed mining in the Tarkine, and the Construction, Forestry, Mining and Energy Union supports the Tasmanian pulp mill.

This puts the unions in direct confrontation with environmental groups, such as the Wilderness Society, who rejected the proposals they saw as “toxic”.

Any successful Labor-Greens relationship will need first to understand these contradictions and then negotiate them. This seems less likely in Tasmania than at the federal level.

The problem for the Tasmanian parties is that they face the perfect storm for a split. A divided population and a reliance on resource-extraction industries, matched with the Hare-Clark system of proportional representation (which regularly leads to minority government), means tensions are almost inevitable. Even if the parties disagree fundamentally, they’re bound together by the state’s parliamentary institutions.

At the federal level, however, minority government was largely a historical accident, unlikely to be repeated in the near future. This means that if it doesn’t repeat soon, both parties will get what they want: room to distance themselves from the other.

Of course, other tensions will remain. Labor will still need to balance the concerns of different social bases. The Greens will likely use those compromises to attract voters on Labor’s socially liberal wing. Similarly, as long as coal mining remains in Australia, the two parties will find it difficult to agree over certain economic and environmental policies.

Ultimately, this bipartisan hunger for distance means that, unlike the right’s Liberal-National coalition, it’s unlikely that we’ll see a formal left-wing alliance any time soon. Each party sees too much to gain in going it alone.