As was widely expected, the opinion polls before Saturday’s election overstated the Greens vote: they failed to reach, or even get close to, the 25% mark. Even so, their 21.5% is a striking achievement. The question for them is what do with it.

Clearly the Greens are well and truly established on the Tasmanian  scene; this is the seventh successive election where they have exceeded 10%. As far as I can tell, it’s the highest third-party vote recorded in  any state outside of Queensland since the establishment of the two-party  system — beating such notable marks as the South Australian Liberal Movement of 1975 (18.3%), the Victorian DLP of 1961 (17%) and the Victorian Country Party of 1945 (18.7%), plus, of course, their own Tasmanian record of 2002 (18.1%).

But it is not a breakthrough to major-party status. Nick McKim, or any other Greens leader, will not be able to go to the next election with any credible aim of forming a government. Although tantalisingly close, they again have failed to win a second seat in Denison, and a party that can only win one seat per electorate will always be a minor party.

For the medium term, therefore, the Greens’ ambition must be limited to being a junior partner in government. McKim has already taken the party to the centre, giving it new respectability; now he needs to demonstrate that the Greens can be trusted with a share of power, and that  majority  government is not necessary for stability.

That’s why an arrangement with the Liberals, hard to imagine in any other state, is a serious possibility. For the Liberals, it’s a win-win proposition: if it works, they get a stable term in government; if it doesn’t, they can go to a fresh election arguing the need to be given a majority. For the same reason, the Greens have a strong incentive to make it work.

It seems that the logic of this situation is starting to dawn on the Labor Party, with the realisation that they cannot assume a Liberal-Greens combination will automatically fall apart. As Sue Neales reported in Tuesday’s Mercury, questions are being raised within the party “about why the government should go meekly into Opposition when it has as much right constitutionally to govern in a minority as do the Liberals.”

After all, if this had been a mainland election, with single-member electorates and Green preferences favoring Labor, the government would  probably have been returned with a working majority. Labor only trails the Liberals by about 2%; in South Australia it trails by 4%, and no one questions Mike Rann’s right to continue in office there.

So Labor may try to evade David Bartlett’s commitment to go into  Opposition (although if Andrew Wilkie scrapes home in Denison, giving Labor a plurality, even that will not be necessary) and reach its own arrangement with the Greens. Since the Liberals have ruled out actual coalition — there will be no Greens ministers in a Liberal government, at least this time — Labor may even consider trumping their offer by agreeing to take the Greens into cabinet.

Failing such a tempting offer, however, it makes sense for the Greens to try to reach agreement with the Liberals, particularly since that seems to be what their voters prefer. It offers the chance to show the electorate that they can be responsible and, just as importantly, to show Labor that they can’t be taken for granted.

With three big elections coming up on the mainland, that lesson may have wider implications.