AAP/Alan Porritt

Greens leader Christine Milne faces her existential moment at the coming election. Her predecessor Bob Brown took the Greens to their present high point - nine senators and the sole balance of power in the upper house. If Milne loses that pivotal position, the Greens will automatically be stripped of a great deal of their clout when the new Senate starts in July next year. It would be seen as the beginning of the minor party’s decline.

The Senate contest is far more crucial to the Greens than what happens in Adam Bandt’s seat of Melbourne.

Milne’s spray this week against the Gillard government, breaking the 2010 alliance, is a strategic move in this fight for continued political relevance. Anticipating an Abbott win, the Greens’ pitch for Senate votes is a more extreme version of the old Democrats’ line of “keeping the bastards honest”. The Greens will say they’d try to stop an Abbott government wrecking the country.

With the conservatives enjoying such strong support in the polls, the electoral task ahead of the Greens is formidable, despite having only three of their nine senators facing the people – Sarah Hanson-Young in South Australia, Peter Whish-Wilson in Tasmania and Scott Ludlam in Western Australia.

Depending how the vote goes in various states, Tony Abbott could have a Senate that allowed him to repeal the carbon and mining taxes with the support of Right leaning crossbenchers.

In the Senate proportional representational voting system, how the numbers play out will depend not just on how the Greens poll but also on Labor and Coalition levels of support.

Of the three upper house Greens up for re-election at this half-Senate poll, the safest will be Whish-Wilson, who took Brown’s casual vacancy last year. Tasmania is Green heartland. Ludlam and Hanson-Young both face tough fights, with the position of Hanson-Young particularly dicey - despite her high profile - because of Labor’s soft vote.

On the other hand, the Greens have a chance of picking up a seat in Victoria and possibly in New South Wales (where their candidate is Legislative Council member Cate Faehrmann). Green sources believe their continued grip on sole balance of power could depend on whether Hanson-Young survives, whether Ludlam can prevent the conservatives winning four seats in WA - or whether (an extremely outside chance) Simon Sheikh, who brings the formidable GetUp! campaigning skills to the task, can snatch a seat in the Australian Capital Territory.

If things go well for the conservatives and badly for the Greens an Abbott government, seeking the crucial 39 votes to pass legislation, could be negotiating with independent Nick Xenophon (who has earlier enjoyed a taste of the balance of power, with his dancing partner then being Family First’s Steve Fielding) and the DLP’s John Madigan. A Katter person from Queensland could also be in the mix. (And who knows what other crossbench candidate might fluke a seat?) It’s highly unlikely the Coalition could win outright control of the Senate.

Milne’s task in appealing to people to vote for the Greens as a Senate check will be challenging because she needs to draw from two constituencies. Most obvious is the Greens hard core base. These true believers just want an Abbott government thwarted. But the second, and potentially large, group (given the polling is showing public uncertainty about Abbott personally) comprises voters who would just like the rougher edges taken off a Coalition government. These are in the middle of the political spectrum, not on the far left of it.

Here arises the argument about “mandates”. An Abbott government elected with a good majority would surely have a mandate for its main, well publicised policies, including repeal of the carbon and mining taxes. But if they retained sole balance of power the Greens would claim they had their own mandate - to stop those policies. This absolutist stand might put off some of those people who would otherwise like a watchdog Senate. The upper house contest naturally is the secondary story in an election campaign. But it can be extraordinarily important. Howard’s unexpected clinching of Senate control at the 2004 election led to WorkChoices, which brought radical changes to the industrial relations system. When that backfired, quite a few Liberals regretted that the Coalition hadn’t been saved from itself by an upper house check.

At this Senate election the stakes will be very high for not just the Greens but the major parties. If the Greens retain sole balance of power and Labor joins with them to block the carbon tax repeal (which would be a big decision for the ALP), there would be a double dissolution. Despite Labor’s claim that Abbott would retain the tax, he would not have a shred of credibility if he did not keep his word.

In a double dissolution the quota is small, so in theory it would be good for the Greens. But if a small party is on the nose, it can be another matter. The DLP was swept out in the double dissolution of 1974, never to be seen in federal parliament again until Madigan’s election in 2010. The Greens have different, more solid, roots but can’t be cavalier about the long term.

So if Milne did retain sole balance of power, the exercise of that power could bring its own big problems.

Postscript

A political operative with a head for numbers has prepared this “poll of polls” on the Greens primary vote. It brings together Nielsen, Newspoll, Essential, Morgan (both telephone and face-to- face), and Galaxy. From a high point of 11.7 per cent around the time of Bob Brown’s resignation in April last year, the trend has been generally down, to 9.6 per cent now.