Cameron is currently between a rock and a hard place. He knows that within the week he could easily be PM if he offered everything he already has plus a referendum on STV versus FPTP. Unfortunately dinosaurs are threatening revolt over the issue that has always been a key Lib Dem policy, perhaps more so than immigration was ever a key Tory policy.
What does he do, offer the referendum in the deal and risk open splits in his party, or not offer it and hope that the Lib Dems accept anyway? Taking Lib Dems out of the equation, perhaps Cameron is relying on the fair votes protests and campaign to build a bit more steam.
There is a simple route forward, and it's Cameron showing his own party pages 66 and 67 of their 2010 manifesto alongside the support that a referendum on PR is gaining. It would be extremely hard for the party to stand against their own claims of involving the public more in the decision making of the country, not if they have *actually* changed to a party ready to let go of the reigns just slightly.
But equally he can say that they have clearly told the public they support FPTP. Any referendum would solely be on the basis of them clearly speaking to the politicians of this country about what they want, it doesn't mean that the Tories are bound to support those in the public that want a change in voting systems.
To me, as much as I dislike the Tories and their policies, this feels like a time for Cameron to show his true colours. Does he believe what he's told people he is, and how the Tories have changed...or will he and his party end up reverting to type before they've even officially got power? In one negotiation Cameron can simultaneously stamp his authority on his party AND likely win over some of the wavering sceptics in the country.
He won't win me over, he'll have to go a whole lot further than just offering a simple referendum on voting reform, but it'd be a very good start to a government that is meant to represent politicians talking to each other and making concessions in order to create stable governance.