Michelle Grattan | Adam Bandt is wedged by Greens' overreach on emissions legislation

The Greens are putting as a condition of supporting the bill - now before parliament - that the government commits to a ban on new coal and gas projects.

They pitched for the ban when parliament was considering legislation for the 43 per cent emissions reduction target, but the government stared them down and they ended up backing that bill.

Now, the stakes are much higher - for both government and Greens.

The 43 per cent target didn't have to be law. That was just icing on the cake. In contrast, the government needs the safeguard legislation - which forces the biggest polluters to reduce their emissions - to implement its policy.

Reform of Australia's emissions reduction regime is at the heart of Labor's agenda. To be stymied on implementation would be a major setback.

From the Greens' point of view, to have failed once to force the government's hand can be brushed over. To fail twice risks making the party look impotent in the eyes of its supporters.

The Greens say they are not issuing an "ultimatum", leaving themselves wriggle room for retreat. But their words are strong, and stepping back would be seen as precisely that.

Just like the Liberals, the Greens have a base that is split between hardliners and moderates. At the radical end, their activists don't want the party to compromise on core issues; in contrast, its mainstream voters want outcomes.

The Greens have history on standing in the way of progress on climate policy, and the government is rubbing their noses in their past. Greens opposition killed the Rudd government's Carbon Pollution Reduction Scheme. Their explanation is that it "was bad policy".

Environment Minister Tanya Plibersek on Thursday said, answering a Greens question in parliament, "when you lined up with the Liberals last time to block the Carbon Pollution Reduction Scheme, what we saw was more emissions for longer".

Of course given that, on an ordinary interpretation of "mandates", Labor has one for its climate policy, the Coalition should let the legislation through - which would make the Greens irrelevant.

But the opposition is spurning any recognition of Labor mandates for core election policies. This deals the Greens (and non-Greens Senate crossbenchers) into the centre of things.

But while holding a whip hand, the Greens are also wedged on the safeguard legislation.

It's hard to see that, at the end of the day, they have anywhere else else to go than to vote with the government. Do they really want to line up with the Coalition (again) to reject a major initiative - to be accused (again) of making "the perfect the enemy of the good"?

Bandt rejected that line on the Voice. Senator Lidia Thorpe defected from the Greens to sit on the crossbench because she thought the party wasn't being pure enough on Indigenous policy.

Thorpe argued a Treaty should be given priority over the Voice. But Bandt, while noting the Greens still think a Treaty should come first, said he didn't believe a "no" vote on the referendum would bring a Treaty closer. It was sensible pragmatism.

Neither would a "no" vote on the safeguard mechanism be likely to bring closer a ban on new coal and gas ventures.

Labor, already facing deepening economic problems, would trash its credibility with investors, business generally and voters if it agreed to the Greens' ban.

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Climate Change Minister Chris Bowen says he is open to negotiation on the safeguard legislation, within the policy Labor took to the election. That provided for new fossil fuel projects to be considered on their merits.

Apart from the issue of project bans, the safeguard bill itself - due for a Senate vote in March for a July 1 start - and the associated draft rule are coming under fire, especially for being too generous on carbon credit offsets.

The head of The Australia Institute, Richard Denniss, wrote in the Guardian: "The reality is the safeguard mechanism does more to safeguard the fossil fuel industry than it does to safeguard the climate. It hides its support for fossil fuel expansion behind a fig leaf of dodgy carbon credits and offsets."

In contrast, Carbon Market Institute CEO John Connor argues the safeguard mechanism reforms, reducing pollution limits by 5 per cent a year, are significant.

"They will send a multi-billion-dollar and growing signal to our largest emitters to drive at source decarbonisation, while requiring investments in emission reductions elsewhere in the economy when they can't do so immediately at the relevant facility," he says.

"With our high-carbon political economy and historic policy convolutions, it would be a major setback to lose the safeguard mechanism reforms," Connor says, although adding there should be some amendments to the legislation.

As he tries to chart his course for exercising the Greens' share of the balance of power in the Senate, Bandt might at times mull on the now-extinct Australian Democrats and their one-time leader Meg Lees.

Lees negotiated a deal with the Howard government for the introduction of the goods and services tax. She extracted concessions for the Democrats' support, and she did the right thing facilitating the legislation. It was a change to the tax system the country needed.

But Lees paid a high price in a party that was divided over the issue, with many of its supporters abhorring compromise. Ultimately, it cost her the leadership.

This is not an argument against Bandt compromising, which he should and almost certainly will have to. It's just a reminder that sensible decisions can impose great pressures on the leaders of minor parties when those parties exercise real make-or-break power.