The Lib Dems, who have had many internal discussions about what kind of deal they would cut, have a whole set of mechanisms—the triple lock—to ensure that the party is happy with any deal that their leader strikes. There are no such safeguards on the Tory side.
A formal Tory Lib Dem coalition is highly unlikely. But some kind of agreement to get a Tory Budget and legislation through is a distinct possibility. In these circumstances, Cameron would almost certainly have to offer some concession to the Liberal Democrats. It is vital that Cameron does not concede anything that his party views as non-negotiable.
The problem for the leadership is that with the chairman of the 1922 standing down at this election and so many new Tory MPs coming in, it will be hard to gauge the views of the parliamentary party.