Late last year, federal Greens leader Richard Di Natale expressed his enthusiasm at the prospect of serving in a federal Labor-Green coalition government. This suggestion has, however, been firmly rejected in recent days by Labor leader Bill Shorten.

But is Shorten unwise to rule out forming a coalition government with the Greens so early in the campaign?

There are four reasons why the Greens are shaping up to be a formidable force both during the campaign and once the outcome is eventually declared.

Political ingenues no more

The Greens are now well and truly part of the political furniture, with a presence at all levels in Australia politics for more than 30 years.

The party has 11 federal MPs, as well as an additional 23 MPs in most Australian parliaments, except Queensland and the Northern Territory.

This brings the Greens certain structural and institutional advantages going into this federal campaign.

First, legislative office gives the party access to state-funded resources that are useful during an election campaign.

Second, it imbues the party with a measure of institutional legitimacy and standing in public debate.

Third, the Greens can point to a growing track record of influencing legislative outcomes, and experience in negotiating with governments.

Importantly, the Greens’ longevity has rendered them a more familiar and less threatening presence. This may make it difficult for their opponents to sustain claims that the party is inexperienced or unpredictable.

Friends with benefits

The Greens have been courting new friends among segments of the union movement – something many commentators once believed to be impossible.

These union allies are of practical and symbolic importance for the Greens.

In financial terms, union donations to the Greens totalled almost A$600,000 in 2013-14. While these amounts are modest compared to union donations to the ALP, they are of critical importance to an otherwise cash-strapped party.

In symbolic terms, union donations can be read as a sign of the party’s growing political acceptance by more traditional social democrats. While complex reasons underpin the decision on the part of unions to donate to the Greens, it nonetheless hints to an emerging rapprochement between the old and new left.

The Shorten factor

Labor has been struggling to rebuild its primary vote. While its difficulties in this regard are structural and sociological, they are also aggravated by Shorten’s unpopularity.

Shorten is not tracking favourably in opinion polls, even if Labor’s two-party-preferred vote is promising.

According to Essential polling, Shorten’s disapproval rating continues to rise, from 27% in 2013 to 47% in March 2016. Over this same period of time, the proportion of voters who approve of Shorten has sunk from 31% to 27%. Meanwhile, the proportion of voters who are “undecided” or “don’t know” about Shorten has declined from 43% to 26%.

The problem for Labor is that studies suggest that public perception of the party leader is important to the outcome of an election.

A decline in Labor’s primary vote often translates into Green electoral gains. It is for this reason that the Greens are quietly optimistic about their prospects in some Labor-held inner-metropolitan seats. The Greens are targeting the Victorian electorates of Melbourne Ports, Batman and Wills. Similarly, the Western Australian seat of Fremantle and the seats of Grayndler and Sydney in New South Wales are also not entirely out of reach.

The Greens’ chances in these seats will be strengthened if the Liberals in Victoria and NSW proceed with plans to negotiate a “loose preference” arrangement with them.

While party stalwarts such as John Howard have warned against this, others, such as Victorian Liberal Party president Michael Kroger, are supportive. Any residual opposition within the Liberals might be assuaged by a Newspoll survey that revealed 47% of Coalition voters are “comfortable” with the Liberals directing preferences to the Greens.

The Di Natale factor

The fourth factor that should help the Greens is the ascension of Richard – I am not an “ideologue” – Di Natale to the leadership.

Di Natale casts a very different shadow from that of his predecessors. His political journey was different from that of former leaders Bob Brown and Christine Milne.

Di Natale appears to have been politicised by social justice issues rather than strictly environmental concerns. He also much more closely resembles an important segment of the Greens base in his style, manner, beliefs and approach. Like many Green voters, he is a well-educated, white-collar professional drawn from the caring/welfare sectors.

Di Natale has sought to re-position the Greens as a mainstream progressive party. This is reflected in his policies, which remain true to the party’s core beliefs while widening the net to draw in other constituencies.

The Greens remain staunchly opposed to offshore processing, and continue to advocate for pricing carbon pollution and legalising same-sex marriage. At the same time, policies such as the “Buffett rule”, which seek to limit the amount of deductions high-income earners can claim, are likely to offend only those who earn more than $185,000 a year.

Similarly, last year’s plan to seek the removal of negative gearing was grandfathered so as to not alienate voters with existing investment properties.

Di Natale’s open appeals to policy moderation may just be enough to motivate those voters who have toyed with voting Green to finally do so.

How might this play out for the Greens?

While the Greens are not contenders for government in their own right, they are important players coming into this election.

At best, they may be needed by Labor to form government. At worst, they should continue to hold the balance of power in the Senate.