Cameron's tactic at PMQs was quite clear, and is now the Tories' strategy of choice in general: "Weren't things so much better under dear old Tony Blair, whom we gave a standing ovation on the floor of the House of Commons, and whose clone is our own Leader, David Cameron?"
If you weren't that slavishly pro-Blair, never mind if you weren't pro-Blair at all, then why on earth would you vote Tory? Not, of course, that you still have to.
And if you were that slavishly pro-Blair, or even pro-Blair at all, then why on earth would you not vote Tory? Blair himself is certainly going to.