As the dogs of war set to barking amid a new round of military intervention in Iraq, not all the stocks that have been rising have belonged to defence companies.

A theme of recent opinion polling has been an uptick in support for the Greens, with the latest weekly reading of the BludgerTrack poll aggregate putting support for the party at 12.1% — 3.6% higher than at last year’s election, and the strongest it has been since that time.

For those who closely follow discussion of Australian politics on social media, this will come as no surprise. In the last fortnight or so, no one with more than a smattering of leftists among their friend- and follow-lists has ever had long to wait before the next denunciation of Opposition Leader Bill Shorten’s timidity and subservience hauled into view on their timeline.

As for the public at large, indications are that Australian involvement has majority support, but only just. Newspoll found strong backing for humanitarian aid and weapons to forces opposing Islamic State, but when Roy Morgan asked respondents about the government’s plan to deploy 600 defence force personnel, the split was only 54-46 in favour. As a mixed bag of results from Essential Research made clear, a lot depended on how the questions were structured.

With an overall picture of a modest but very-far-from-overwhelming balance of support in favour of the government’s position, the issue amounts as a wedge for Shorten and Labor, regardless of the extent to which the government has designed it as such.

Shorten’s calculation that his interests are best served by keeping debate focused on domestic issues is no doubt well founded, but it is not without its costs. Prime Minister Tony Abbott has been granted clear air in which to play to a natural advantage on defence and national security, which has loomed large in the government’s modest recovery from the depths of the budget slump.

Labor’s position has also, if the military metaphor may be excused, exposed its left flank to the Greens, who face no competition for the very substantial constituency opposed to intervention (the odd Labor dissident notwithstanding).

For all concerned, this is a familiar state of affairs — a fact that emerges through observation of Greens polling over the long term, as detailed in the chart below.

A recurring theme is that the fluctuations in Greens support say more about Labor’s activities of the time than anything under the party’s direct control. When Labor cleaved to the Howard government’s line on asylum seekers during the Tampa episode in 2001, the Greens doubled their national vote at the election held a few months later, laying the groundwork for their takeover from the Democrats as the dominant force in minor party politics in 2004.

Conversely, the poll surge to Labor after Kevin Rudd became leader in December 2006 hit the Greens as well as the Coalition, in part because Rudd’s campaigning on the Kyoto protocol made an election issue of climate change on Labor’s terms. When the Rudd government shelved the emissions trading scheme in early 2010, support for the Greens hit double figures for the first time — and shot up further still two months later, when Labor supporters dismayed by Julia Gillard’s leadership coup sought a vehicle for a protest vote. Gillard’s otherwise troublesome association with the carbon tax took some of the gloss off the Greens over the following term, compounded by a clear drop of nearly 2% when Bob Brown resigned in April 2012.

The present lift in support for the Greens is the second of its two spikes this year, the first being a very short-lived effect of Scott Ludlam’s outstanding success at the Western Australian Senate election in April, with no small help from Labor candidate Joe Bullock. While the present Greens surge is likely to be similarly transient, it forms part of a clear picture of improvement since the start of this year.

However, this doesn’t seem to be doing Labor any harm, with the two parties sharing equally the benefits of the decline in support for the Coalition. According to the poll aggregate, Labor has held its own on the primary vote over the past few weeks, with the Coalition losing ground for the first time since its recovery in the wake of the MH17 disaster.