Perhaps the obvious thing David Cameron could do is go for the idea of a referendum. In theory that would buy off a lot of Tory rebels. In theory, it would perhaps get the Labour Party on board or give them fewer reasons to object to it.
In theory you would have all three party leaders presenting a united front which the polls tell us broadly support Lords reform.
The difficulty is that Nick Clegg doesn't want this. He fears it will be like another AV referendum, that a lot of the Tory rebels will turn on him and that the country will take it out on him and reject it.
Most people don't know how David Cameron is going to get out of this. If he doesn't the Lib Dems have made it very clear they are not going to support boundary changes.
If they go ahead with that threat their ministers would probably have to vote against what is a government proposal and I think that would end the coalition.