Ministers are well aware they are lumbered with what one elegantly called a "shit sandwich" - selling a pay cut for the lowest paid as a pathway to more jobs. At best, this is counter-intuitive, and for many, just unconvincing.

Even worse, it is complicated.

Bill Shorten, on the other hand, has a neat, uncluttered argument: The Coalition plans to give big business a $50 billion tax break but weekend casuals will get a pay cut.

And where Turnbull parades abstract economic modelling, Shorten counters with real-life victims.

With the ACTU gearing up for a WorkChoices-style campaign, pitching the cut as the thin end of an ideologically driven wedge against workers, it is game over, you'd think.

And you'd be right . . . mostly.

But in Parliament on Tuesday, there was a sense that the PM is warming to this presumably unwinnable fight. Almost uniquely, he feels he has Shorten in a credibility bind over his past promise to respect the umpire's decision.

This remember, was Labor's own industrial machinery. Plus, there's the historical absurdity of Labor undermining the "IR club" while the Coalition defends it.

In Parliament, Turnbull pointed to previous enterprise agreements entered into by major unions, like the all-powerful SDA or shoppies - he called them the "shoppos" rather embarrassingly - which cut weekend penalties while increasing pay for permanent employees. In other words, deals that saw casuals cop it in the neck, so full-timers could benefit.

Such agreements suggest unions are not always as high-minded about vulnerable - read less unionised - casuals as they would have you believe.

Labor's confidence in this argument is well placed. But Turnbull is determined to lay bare Shorten's contradictions for all they are worth.

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