I suspect Rudy's great sin to some of these people is that he might actually win. Some of our brethren don't want to win. They just want to go off to a corner and snipe. After all, it's so much easier, and more gratifying to certain kinds of warped egos. They resent anyone who even reminds them that victory is an option.
I don't think Rudy would do nearly as well in the general election as his supporters seem to think. The number of moderate voters Rudy grabs now probably exceeds the number of conservatives he's leaving behind, but how many of those moderate voters will he keep after the Democrats start actually campaigning for them?
One major advantage of working to secure voters in the "base" is that the Democrats won't be competing for those voters and consequently one won't have to expend scarce resources trying to hold onto them.
His great 'sin' is actually a bunch of sins, and those sins happen to be his liberal positions on lots of issues.
Simply put, he would split the Republican Party in half should he win the nomination.
Some of our brethren don't want to win.
If we have to win by nominating a Republican liberal, then what have we won?