Within the week, they've become the Famous Five. Four gay men and a colourful, tough-talking straight guy from north Queensland who for years has held aloft the rainbow banner.

Their cause is same-sex marriage, their modus operandi the crash tackle. At one level, what they are trying to do is simple. They want the Government's commitment to a plebiscite on same-sex marriage ditched in favour of an early free vote in parliament.

But the push by the five Liberals has unleashed an existential battle that goes to the heart of Malcolm Turnbull's leadership. It has consumed the Government for days, mobilised the Liberal conservatives, and raised talk of a National possibly defecting to the crossbench — which would wipe out the Government's majority.

Even acknowledging that Australia is lagging internationally and that same-sex marriage should have been delivered yesterday, the implosion around the issue is extraordinary.

PM in a quagmire

Turnbull has been left flat-footed. All week he has been out and about in Western Australia, where polling shows the Coalition could lose several seats at the next election and the state's GST revenue deficit is a source of bitterness.

Turnbull's public responses to questions on the crisis over the marriage issue have been dismissive; he was irritated at being asked. Whatever he might be doing behind the scenes, his public stance is one of following, not leading.

Malcolm Turnbull should have seen this week's test of strength coming. ( ABC News: Jacob Kagi )

On Thursday he called a special Liberal Party meeting for Monday afternoon, a day before Parliament begins its spring session. Options before MPs range from sticking with the plebiscite policy, through having a postal vote, to wiping the slate and settling the matter in Parliament.

While the Liberals will meet separately, Turnbull pointedly noted that "ultimately Coalition policy is determined in the Coalition partyroom". In other words, both parties — as a collective — need to end up on the same page, in his view.

As he struggles in this quagmire, memories of that 2009 brawl over carbon pricing which triggered his demise as opposition leader must niggle at his mind. Then, however, he was digging in behind his belief. It's not so simple now.

Inconsistent stance

Tellingly, the row is exposing Turnbull's penchant for inconsistency. Everyone recalls the 2015 row over same-sex marriage, when then prime minister Tony Abbott was thrashing about and Turnbull was spruiking a parliamentary vote.

Amid the present talk of a possible postal ballot, there surfaced a 1997 Turnbull article, written in another context but with all his characteristic passion, that declared voluntary postal voting "flies in the face of Australian democratic values".

Usually it is oppositions that are more vulnerable to ill-discipline. But this Government too often resembles a pack of fighting dogs. A little while ago its members were scrapping over energy policy. Now that battle is temporarily on hold, while they turn their energies to this one.

Turnbull caught by surprise

The test of strength that exploded into public view on Monday has been coming for months. There was a plan to bring it on in the last parliamentary session. When that didn't happen the talk — which was reported — was that it would probably come back in the spring session.

So Turnbull should have been prepared. Instead he looked to be taken by surprise.

Backbench revolts occur even in the tidiest of governments but circumstances make this one more dangerous than most.

Same-sex marriage has become a core issue for the five; at the same time, it is totemic for the hardline conservatives, and also a symbol of their wider discontents with Turnbull.

It strikes all the harder at Turnbull's weakened authority because he is defending the plebiscite position he condemned just two years ago. The imbroglio is exposing him as a man who compromised his principles to win leadership and now can't manage his party or, for that matter, his faction, because most of the advocates for change are moderates.

Famous Five and the tough choice

If the Government lands on the postal vote (perhaps preceded by putting the plebiscite legislation to the Senate again) as the way ahead, the Famous Five face an invidious choice. Do they reluctantly accept as a step forward what for them personally would be a humiliating step backward?

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 8 minutes 7 seconds 8 m Emma Alberici speaks to Warren Entsch about the upcoming meeting.

Or do they point to its flaws and go ahead with their private member's bill, which has been spearheaded by Western Australian senator Dean Smith, one of their number?

If all four who are in the Lower House pushed on in Parliament, the numbers would likely to be there to get the issue to a substantive vote.

Of the four, Warren Entsch, in what is expected to be his last term, has the least to lose. For the other three, Victorian Tim Wilson, Queenslander Trevor Evans, and Trent Zimmerman, from NSW, the stakes are much higher. They are early in their parliamentary careers. Defiance of the party would bring a huge blowback.

They might land their cause, putting their crazy brave stand into the history books, but with consequences to themselves, Turnbull and the Government that are — at this point — impossible to predict.

Opposition focused on policy

The contrast this week between a riven government and a united, policy-centred opposition was dramatic.

Bill Shorten might have been a little nervous about Labor's Sunday announcement of a crackdown on family trusts. But the Government's counter-attack, difficult anyway because the Labor measure is relatively mild, was overshadowed by the Liberal infighting.

Bill Shorten's week has been focused on policy initiatives. ( ABC News: Nick Haggarty )

Labor, with a policy that says it would legislate for same-sex marriage in its first 100 days, is waiting and wondering about the implications for it of the barney within the Government.

If a postal ballot resulted in a "yes" vote, that was followed by a bill bringing in the change, Shorten would have lost a useful issue at the election — as he would if the five delivered an early parliamentary result.

For most people same-sex marriage is not a vote switcher. Those for whom it is would probably not be voting for the Coalition anyway. So, if legislation were passed this term, it would be unlikely to turn votes Turnbull's way.

But the issue is a rallying point for Labor, projected as a future achievement for an ALP government. Don't totally believe Shorten when he says he'd like to see same-sex marriage as soon as possible. He'd prefer to see it when he delivered it.

Michelle Grattan is professorial fellow at the University of Canberra.

This article was originally published in The Conversation.