We can sell swing voters on strong defence and fiscal discipline, but whenever we try to sell them a regressive social agenda, they tell us to go fuck ourselves.

In what way did John McCain try to sell a regressive social agenda? Bottom line is, McCain is exactly what you describe as a centrist moderate with regard to social issues. On the flip side, with regard to fiscal issues, he’s a centrist moderate that doesn’t happen to be quite as far left as Obama. IMHO, if you want to appeal to the independent middle (and I’m among them) you’re going to have to focus on fiscal issues.

McCain is evidence that you can be socially moderate and still have no appeal to the middle for what should be an obvious reason: reckless federal spending. But if it makes you feel better to blame social conservatives for all of your problems in winning elections then carry on. The results speak for themselves. As much as you want to blame social conservatives for your problems as a party, the reality is that you need their votes to win elections.