The Work and Pensions Secretary took a break from invoking the spectre of feckless hordes on jobseeker's allowance or shysters blagging sickness benefits - only to go after those who did have jobs instead.
And not just any workers, either: those on wages of £13,000 a year or less, whose low pay entitles them to housing benefits and tax credits.
As with any sliding-scale scheme, if their pay shrinks even further on any given week, those benefits are bumped up in order to make up the difference.
But once they amalgamate into universal credit next year, Duncan Smith said, he plans to give payroll departments the power to report workers on strike to their local DWP branch - who will then be obliged to freeze their payment rates.
The minister - who boasts an estimated personal wealth of £1 million, largely from ritzy speaking engagements - insisted he could not stand by and see such undeserved riches strewn before them.
Not out of malice, or to squeeze another million from the welfare state, or as a sop to unscrupulous employers, you understand - the very idea was morally wrong, creating "perverse incentives.
"Striking is a choice, and in future benefit claimants will have to pay the price for that choice - as, under universal credit, we no longer will."
His Sunday sermon seemed to have caught the country's trade unions unawares, with the Morning Star unable to print several officials' initial response.
But if he makes good on his threat, the policy could strike a death blow for meaningful organised labour in Britain.
In the finest traditions of Whitehall, let's analyse policy with a thought experiment and only the most favourable statistical data.
So, our prospective picket earns the upper limit of £13,000 a year and is employed in a single full-time job in the economic hub that is London, nine-to-five, Monday to Friday.
Presumably they work in the same mythical place where health and safety regulations require welding masks for playing conkers, but forget that for now.
That's a take-home pay of just £6.25 an hour, or £218 a week after tax.
Assuming again that they have no dependants to worry about, under universal credit that means a budget of £225 a week to cover rent, travel, fuel, groceries and all the rest of it.
£6.25 an hour is coincidentally the same amount the cleaners at Buckingham Palace were taking home before the civil servants' PCS union led a campaign demanding the London living wage of £8.30 an hour.
Public pressure over the royal wedding and looming jubilee forced the Queen's contractors to buckle, conceding a rate of £7.50.
But imagine how a union branch in Buckingham would have fared under Duncan Smith's diktat: for the worker we dreamt up, a vote to strike would have meant losing £45 from their weekly budget for every day spent on the picket lines. That's 20 per cent of a household budget that is already barely scraping past the national poverty line.
Ironically Duncan Smith's call for fairness flips the sliding scale on its head: the less you earn, the more difficult his department will make it to improve your lot.
The policy effectively gives bosses the ultimate leverage: if you are too poor to get by on pay alone, they will make sure you won't get by on anything else. Such power saps any chance of a ballot for strike action, leaving those who represent Britain's most vulnerable workers with nothing more to offer than toothless "service unions."
Admittedly the prospect may not keep the minister up at night, but his moral dilemma should. By the same logic any company which lays off staff or makes them redundant should face a proportionate rise in national insurance employer contributions, since they have made a conscious choice to move people onto the public purse.
Of course such a notion would be laughed out of Whitehall. It's absurd, the Treasury mandarins would say, to expect the economy to improve while deliberately driving those already struggling to get by into an inescapable cycle of debt.
To be fair to Duncan Smith, it's not a perfect analogy: companies can dissolve.
People starve and freeze and steal and die.
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