The 2008 Democratic primary took a long time to wrap up, too, and Obama still won... and so therefore (thus goes the argument), the prolonged GOP primary this year should not be a deal-breaker for the Republicans in the general either, right?
Two problems with that:
First of all, the reason it took so long to converge on Obama over Clinton is that both were excellent candidates. Now, maybe some of this is my own personal biases; I am obviously going to say more positive things about a liberal than a conservative. But I don't recall major mainstream news outlets saying, "Hillary? Are you fucking kidding me?!?!?" And yet that's exactly how the chattering classes are reacting to Santorum's ascendancy. There is broad bipartisan consensus that Santorum is a complete wackaloon, and a truly awful choice for the 2012 GOP presidential candidate.
It would be one thing if, say, it was a prolonged fight between Romney and a younger McCain. I'm not fans of either, but I can objectively recognize they are workable candidates. They are both eminently electable. That Santorum still has a dog in this fight (a man-on-dog, perhaps?) is a fuck-up of major proportions. It does not bode well for the GOP establishment's ability to effectively coordinate their actions in the coming months.
The other thing to remember is that up until pretty close to the general election, the 2008 contest seemed as though it could have gone either way. Maybe the prolonged fight with Clinton did hurt Obama's chances. I think the McCain campaign really threw it away when they signed on Palin as the veep candidate. I totally understand their reasons for doing so, but either it just wasn't well thought-out or they did no more vetting than simply reading a resume. Sure, Tea Party types love Palin, but her complete inability to handle herself in a standard interview, let alone a debate, was bound to make her a laughingstock. The kind of candidate who chokes on a softball question like, "What kind of newspapers do you read?", I mean... that type of candidate is just not going to hold up well to the national spotlight.
The GOP shot themselves in the foot late in the 2008 campaign, and they might very well have won the presidency if they'd picked a better VP candidate and been able to sustain the campaign for the long haul. It's doubtful that the Obama campaign will do something so stupid in the lead-up to this November.
The Republican Party is in sorry shape right now, and everyone can see it. Parallels to the 2008 DNC primary are tenuous at best. Getcha popcorn...