Perhaps (like so many times before) he didn't think it through. Or maybe he thought a cheap shot would earn him a laugh or two.
Doesn't look so clever now, though. The Guardian website reports:
The Conservative MP Nadine Dorries has warned the party's leader, David Cameron, that he will be kicked out of office by his own backbenchers within the next year unless he "dramatically" changes the party's policies.
Dorries described the prime minister and his chancellor, George Osborne, as "dangerous", saying they were leading the Conservatives "towards defeat" at the next general election.The MP for mid-Bedfordshire hit out at Cameron's "sneering" attitude and claimed backbench discussions about introducing a motion of no-confidence against the leader were already taking place following a dismal display at Thursday's local elections.