Malcolm Turnbull's heart must've sunk when he read that John Howard would be teaming up with Tony Abbott for the campaign against same-sex marriage.

The Prime Minister knows what it's like to be at the sharp end of a popular cause, only to see it blunted and then broken by the Howard-Abbott duumvirate.

As chairman of the Australian Republican Movement almost two decades ago, Mr Turnbull was up against Mr Abbott who led Australians for Constitutional Monarchy with the encouragement of then-PM Mr Howard.

Like same-sex marriage now, polls at the time variously suggested an Australian republic either had majority support or at the very least more support than opposition.

But the Yes vote lost in the 1999 republic referendum, as republicans splintered over the preferred model and in the face of what Yes campaign director Greg Barns ruefully regards as simplistic and misleading claims.

Mr Barns said this week that if the same-sex marriage advocates split like the republicans did in 1999, they face a similar fate.

"They will lose if there's disunity and if they fail to counter what has already started to be an outrageous fear and scare campaign with blatant lies," Mr Barns told The Australian Financial Review.

"Disunity and an inability to counter these misleading claims Abbott is throwing around will defeat what should be a no-brainer."

Abbott's culture war over values

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 6 minutes 4 seconds 6 m Malcolm Turnbull tries to switch attention to power ( Andrew Probyn )

Mr Abbott, the most lethal political wrecking ball of his generation, unofficially launched the No campaign on Tuesday.

There was no bunting, no stage or fancy backdrop, just a few devastating lines delivered at the House of Representatives doors to a handful of reporters.

"I say to you, if you don't like same-sex marriage, vote no. If you're worried about religious freedom and freedom of speech, vote no, and if you don't like political correctness, vote no because voting no will help to stop political correctness in its tracks," he said.

Religious freedom. Freedom of speech. Political correctness. Mr Abbott framed the fight against same-sex marriage as the generational culture war over values.

He calls for dissent against the progressives' entire agenda and rails against the Left's encroachment of tradition and institution.

His fight against same-sex marriage is, in effect, his life's work. And he suggests the battle against legalisation of same-sex marriage is a crusade for liberal democracy and not against it.

And he praised the Government's pursuit of the postal plebiscite, which instantly worried some of his pro-gay marriage colleagues, recognising that Mr Abbott had seen instant opportunity to wage successful war.

Just as Mr Abbott distilled a No vote in 1999 as the democratic defence against the "politicians' republic", he celebrates the postal plebiscite on same-sex marriage as a strike against the "politicians' vote".

It is a typically Abbottesque inversion that depicts as undemocratic the very democratic institution of which he is member.

Shorten declares his hand

Sorry, this video has expired Bill Shorten labels same-sex marriage postal vote "ridiculous waste of time and taxpayer money".

For more than two days, the Abbott articulation was left unchallenged by other political leaders.

Indeed, for most of the week it looked like no-one in Australia's political leadership, either in the Government or the Opposition, would take a hands-on role in the Yes campaign.

Mr Turnbull remains hesitant, neither wanting to take ownership of a deeply flawed process nor wanting to inflame the right who have long warned of giant conflagration over the gay marriage push.

The PM said he would vote yes but said other prime ministerial commitments would keep him from active campaign; it was unconvincing and meek.

Opposition Leader Bill Shorten, strung between a process he detests and an outcome he desires, took until Thursday to declare his hand.

When it came, Mr Shorten's declaration was strong, passionate and barbed.

"I will be voting yes. I will be campaigning for a Yes vote," he told Parliament on Thursday.

"We say to young Australians who are gay, we are voting in this survey, we are participating in this survey because of you.

"Not because we respect the process, but because the Labor Party will not let gay Australians and young gay people cope with this survey, this evaluation of their relationships, on their own."

He said he would hold Mr Turnbull personally responsible for "every hurtful bit of filth" unleashed by the postal vote, having shown "moral cowardice" by licensing the debate.

Vote could ruin Turnbull

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 7 minutes 20 seconds 7 m Government to spend at least $120m on postal plebiscite ( Andrew Probyn )

It was a condemnation of character so personal it will invite retaliation. The Government will step up its "Kill Bill" strategy in weeks ahead to offset the brand damage done to the PM on the same-sex marriage debacle.

While Mr Shorten pledged to campaign for the Yes campaign, his insurance policy is in blaming Mr Turnbull for failure. Rather than ownership of the debate, Mr Shorten has taken a short lease.

The PM has no such political insurance, yet he needs the Yes vote to win. His allies know this.

And yet by aligning himself alongside Mr Shorten, Greens leader Richard Di Natale and gay rights advocates, Mr Turnbull again risks setting himself against the majority of his partyroom.

But think what a No vote in the postal vote would herald: Liberal agitators, already dismayed by their leader, would seek to bring on a private member's bill to legalise same-sex marriage.

The conservatives would revolt, accuse their colleagues of ignoring the popular view. Arguments about legitimacy of the vote would ensue. Tony Abbott would be at his destructive peak.

Mr Turnbull would be ruined.

And with the nation reeling that a popular cause had been brought unstuck by a process of connivance there'd be a rush to steal the line Mr Turnbull once used against Mr Howard after the republic debate:

Malcolm Turnbull would be the prime minister "who broke the nation's heart".