The darling of the backbenches, Carswell has been a thorn in Cameron's side for a while now.
Peace seem to have broken out when Carswell appeared to back the PM over his pledge for an in/out referendum should the Tories win in 2015.
However, something has changed in Carswell's mind. Either he thinks the party won't win in 2015 so has decided to jump now or this is some sort of weird stalking horse effort.
If Carswell wins for UKIP in Clacton, it could pave the way for other Tories to follow suit and persuade more traditional Tories to abandon the party for UKIP.
You might expect the Tories to respond, remind voters Carswell appears to have reneged on his backing for the party.
Instead the attack dogs were sent out online to attack Ed Miliband for 'not saying anything about Rotherham'.
We'll ignore the fact the party has called for the PCC to step down, that senior council figure has already resigned with more expected to follow....
And that sums up the Tories, when there is discord within the party, they lash out at Miliband and Labour. Carswell didn't defect because of Miliband, he defected because of Cameron and Osborne.
The killer line in Carswell's interview on Sky was the use of the word 'honest' in relation to Miliband's position on the EU.
The implication being Cameron isn't and that, whether the Tories want to admit it or not, is the Tories biggest headache.
If Cameron doesn't 'get it', then I'm afraid his dreams of an EU referendum will turn to dust.