And under the party's rules, that's enough to force a vote of confidence in him.
It means that on Wednesday, all the 165 Conservative members of parliament will vote on whether they reckon Mr Duncan Smith is up to the job.
If he doesn't get the support of a majority of them - in other words at least 83 people - that's it, he's lost the job.
That would mean another election - for a new party leader.
Told you it was complicated!