There is some recent history to back up this assertion. This, presumably was the calculus behind McCain’s decision to pick Sarah Palin instead of Tom Ridge or Joe Lieberman — and (as Klein notes) probably helps explain some of Romney’s gaffes.
But it’s worth asking whether or not the assumption that this is some sort of immutable law is correct. In the wake of the Bush presidency, and because of John McCain’s long and troubled history with the conservative base, McCain couldn’t afford the luxury of “moving to the center” after winning the nomination. And this probably hurt his image. It’s hard to be a contrarian maverick and suck up to the base at the same time.
But think of the timing. Having won two previous presidential elections (2000 and 2004), and having soured on George W. Bush’s “big government” conservatism, Republican base voters were in no mood for apostasy. But times have changed. Having endured two losses to Barack Obama, and staring at the very real prospect of eight more years of Hillary, one wonders whether or not conservative voters might be a bit more chastened come 2016.