McCain is the target of so much invective because he represents a threat to neo-cons who took control of the Republican party, thirty years ago. He isn't their candidate. They can't rely on him to promote their interests. hell, he represents old school republicans, the ones who don't share the neo-con agenda, but vote for the party from loyalty, naivety or disgust with the candidates offered up for sacrifice by the democrats.
The venom being aimed at McCain is the result of the neo-con fear that a Mccain presidency may lead to them losing Republican party, which they have successfully colonized and controlled for the last three decades. Mccain's an old style Republican, with more in common with past presidents like Eisenhower and Nixon (though hopefully not the corrupt, paranoid megalomanic aspects) than with Reagan or Bush. The neo-cons fear that if he gets the nod, him and his ilk will try to reclaim their party from the evangelists of neo-liberal madness.
They'd rather hand the Whitehouse over to the Democrats than lose their grip on the Republican Party. In fact, in the long term, a defeat for McCain would serve their ends quite nicely - it would shatter the old-style Republican camp, as their candidate had failed, and strengthen the neo-cons, who will be able to say, "I told you so" and re-emerge strengthened from the defeat of the moderate wing.
1 - 'I'll campaign for Hillary if McCain is the nominee,' comments by Ann Coulter, in an interview on Fox News, 5th of February, 2008. Available on you.tube. (http://www.youtube.com/watch?v=HuTqgqhxVMc)
2 - 'The Great Betrayal,' by Patrick J. Buchanan, writing in The American Conservative, February, 2008. (http://www.amconmag.com/2008/2008_02_11/buchanan.html)