Could a cartel of pool operators collude to 51%-attack the blockchain and/or change the protocol?

I remember when GHash.io had around 50% of the hashpower under its control, and individual miners redirected their hashpower to other pools to prevent GHash.io from attacking the blockchain.However, now that mining pools are running more and more of their own machines, I'm wondering whether some number of mining pool operators can collude to attack the blockchain using just their own machines.What impact does the centralization of Bitcoin mining have on the long-term security and stability of the network? via /r/Bitcoin http://bit.ly/1JEwt71