Shadow foreign secretary William Hague said they agreed economic stability would be key to any agreement.
Remember "economic stability" is a code-word here for, "reassuring the markets" (because who else out there is creating instability *except* the markets?)
This, in turn, means, "demonstrating to lenders that repaying them takes priority over taking care of British citizens" which, in its own turn, means "making dramatic, theatrically visible, cuts to government services".
For the Lib Dems to agree that economic stability is key, therefore, means for Liberal Democrats agree not to challenge or vote against Tory cuts to public services.
What the Tories are offering, to buy this quiescence isn't clear. They can't really offer Proportional Representation. Even offering a pathway towards it (such as a referendum) is a anathema to the majority of the party.
So, one of three things is about to happen :
a) the Tories are going to sucker the Lib Dems. They'll tell something to Clegg that he thinks is genuine progression towards PR. But then find a way to avoid delivering it.
b) Clegg is going to wimp-out. He's going to, knowingly accept, some vacuous promise from Cameron, out of fear of looking obstructive.
c) An attempt to form a Lib / Con coalition is going to fail.
Of course, like everyone else, I have no idea which it will be.