If the economy is without question the biggest substantive issue of this election, then electoral reform could be the biggest strategic issue. It is the present system that is still delivering significant numbers of seats to the third placed party, and which threatens to deprive either possible winner (Conservatives, still the favourite, or the resurgent but stabilising Lib Dems) of a governing majority. And Nick Clegg has managed to set the PR hare running with his comments that he wouldn't deal with a third-placed Gordon Brown, whatever the number of his seats. These comments have set David Cameron and the Conservatives spinning furiously about PR - no, they absolutely wouldn't accept it, says Cameron, but maybe a referendum is not out of the question? Iain Duncan Smith on the Today programme has already indicated the sort of pressures Cameron will be under if he even looks at the possibility of a deal with the Lib Dems, where PR would be the first issue on the table. Cameron may be ready to deal, but is he ready for the rebellion in his own party if he does?