The American Spectator's J.P. Freire cites the example of Rudy Giuliani, who was reasonably well tolerated by the Republican primary electorate until the specter (sorry!!) of an old sex scandal did him in. That's proof to Freire that the party has room for moderate Republicans who are nonetheless loyal Republicans.
Still... Giuliani, won 1 delegate to the GOP convention and had to pull out of the New Hampshire primary.
A better example might be Governor Jodi Rell of Connecticut who doesn't fit into J.P.'s convenient (for his purpose) arch-conservative prism. Rell supported stricter gun laws in CT. She has signed tax increases including the reinstatement of the "death" tax. She didn't grandstand about not accepting the stimulus.
Or how about Mitt Romney. The centrist Mitt Romney Massachusetts elected in 2002 couldn't win re-election and still be as conservative as he felt was needed to win the Republican Presidential Primary. So he opted out.
Or Collins. Or Snowe. Or..
The GOP has basically been eliminated from the Northeast by the purists so there aren't many great examples.
Contrast that with how Democrats treat people like Mark Prior, Jon Tester, and Ben Nelson. These Senators aren't anywhere near ideal for Democrats, as a quick glance at the Netroots sites will affirm.
But they don't have to fear primaries by party mutilators like Pat Toomey.
Why do Dems treat their Dogs differently
Is it a function of power? Or party traditions?