This speech was one of the biggest moments in Bill Shorten's quest for The Lodge

Bill Shorten says Prime Minister Malcolm Turnbull will be personally responsible for "every hurtful bit of filth" unleashed by public debate on the same-sex marriage postal survey, while confirming Labor will pull out all the stops and campaign for a "yes" vote.

In a passionate speech after question time - which drew a standing ovation from his Labor colleagues - the Opposition Leader put to bed suggestions the federal ALP could boycott the vote and urged Australians who support same-sex marriage to vote in the non-compulsory survey.

If you're looking for the reason the PM suddenly declared himself a strong leader this week, here it is.

It is a contrast all the more stunning for the fact that both men are personally modern, and both believe the question of marriage is for individuals rather than the church or state.

That the government is in such a muddle on this is hardly new, even if this week it took its desperate manoeuvreings to next-level ridiculous.

The credibility of what is now called a survey – even as a measure opinion - is shot, not to mention the other problems with how it will roll out.

In cold electoral terms, Shorten can't lose and Turnbull can't really win.

Why would a government under extreme pressure, license a three-month campaign just to allow the opposition leader to look like the good guy?

For Turnbull, the worst outcome would be for the postal "survey" to return a negative vote. He would be honour-bound to deny a parliamentary debate, but the issue would not go away, potentially consuming another whole year. Reformers would merely point to the survey's manifold franchise problems, its "wrongness" in principle, and the immutability of their case for change.

He needs an affirmative result. Yet he has already revealed he will not be actively campaigning for that. Why is not clear. Is it that he will not risk inflaming right wing opponents in his party room? Or that he too feels the proposal may go down?

In any event, it makes him look equivocal meaning a 'yes' vote would be more clearly a win for Shorten. In perhaps his single best speech as Labor leader on Thursday, Shorten announced he would campaign strongly for reform and urged others to do so as he scythed into Turnbull's fog, declaring him weak, unprincipled, and hostage to party room ideologues.

A toxic dynamic is already emerging. Now that Shorten and Tony Abbott have so clearly stated their positions, Turnbull risks being sidelined by a much clearer fight between an ascendant opposition leader with public opinion on his side, and a rampaging Abbott intent on internal havoc and recreating the 1950s.