The thing is that I see a lot of allegation of fraud on this site. Heck, in the pats 24 hours I've seen multiple posts saying they have proof of such fraud. I, personally, don't think fraud is happening on such a large scale as people believe it is- despite what people here think, most of the Republican Elite still don't take Ron Paul seriously as a threat. Heck, even during the Iowa Caucus they were more worried about 'ruining' Iowa's credibility than Ron Paul's victory (and this was repeated in private just as much, if not more, than in public). I don't think they consider the libertarian strain of the party enough of a threat yet to resort to absolutely desperate measures.
You have to remember that the GOP has a lot of foes (as does any major political party in the world). They got hold of real, significant pre-determined fraud, the GOP would be ruined. Absolutely shot. Thus, only in the most dire of situations will they resort to such a thing. President Obama would love to find proof of Republican fraud. The liberal portions of the media would also love to find it, as would a large chunk of the American political (and even a decent part of the non-political) blogosphere, and, heck, the internet in general. As I said above, major parties have major enemies.
If they aren't finding it, there is an extremely good chance it doesn't exist. And if you guys really think you have found some evidence, show it to a (liberal) news media outlet. If it really is proof, they will blast it up to the high heavens. If the GOP folds, the Democrats could easily hold onto all positions of power for two or maybe even three decades- don't you think the Dems would kill for that? And make no mistake- pretty much the worst thing an American political party could do was deliberate fraud. It would be seen as unforgivable by the general population. And that is because it isn't tied to a single person- Nixon might have been hated, but he wasn't the GOP. He was just one man. If there is evidence of significant fraud across multiple states (if it was just, say, Maine, then it could be blame on the person running that system, rather than a systematic thing. It would still hurt the GOP's credibility, though), that would confirm that there is a conspiracy, and that could easily destroy the GOP.