There are many reasons to vote against McCain, but the main one is that he will lose to both Obama and Hillary. McCain would lose the general for the following reasons:
1) McCain is old and looks it. He'll be 72 in August 2008. Obama will emphasize youth and charisma, he' ll talk of change and McCain will have no counter. Kiss the youth vote Goodbye. He'll even makes Clinton look young and fresh.
2) McCain comes off as angry and cranky. Remember Bob Dole? Usually the most likable candidate wins, and McCain is NOT likable.
3) McCain doesn't play well with others. He even boasts about it. Like Dole, McCain political staff has had a large turnover. And McCain has alienated many of his fellow Republican congressmen and the Bush Administration. Even "Maverick's" need friends and support when running for the POTUS and McCain wont' get it.
4) Conservatives will stay home. Sure many will vote for him, but without enthusiasm. Others will go 3rd party or not vote. Conservative grass root activists not only vote but they campaign door to door, talk to their friends, neighbors and co-workers, and remind/help people get to the polls. They won't do this for McCain. The base will be further depressed because McCain dislikes talking about Social issues and many Evangelicals remember his hate filled rants against Falwell and Robertson.
5) Reagan Democrats won't vote for him. Low and middle income whites in the Midwest and Florida have voted Republican based on social issues and national defense. The Republicans win over them by attacking the democrats as "Card carrying member of the ACLU" or unpatriotic and weak on defense. For example, Bush owes his victory in Ohio in 2004 to anti-gay marriage voters. But McCain doesn't care about social issues, so that's off the table and he dislikes attacking Democrats. So, that leaves Iraq and the WOT however, people are tired of Iraq. Further, McCain's dislike of border security and torture undercut any attacks on The Democrats as weak on the WOT. Finally, McCain's almost hysterical support for foreign aid, "free-trade", NAFTA and amnesty will hurt him with this group.
THE MOST IMPORTANT POINT.
6) McCain won't adequately respond to future attacks from the MSM and the Democrats. McCain has been given a free ride by the MSM for almost 10 years. Remember the 'Straight talk express'? Even the liberal democrats like him, Kerry even asked him to be V-P. As for the press, Matthews has a man-crush, Russert loves him, Brownstein's wife works in his office. The love affair is so intense, even the press jokes about it. Except for a few tough questions on Iraq, the press builds him up and never tries to tear him down.
This will all end when McCain in nominated. The love affair will end. We will hear about the Keating Five. His first marriage will be discussed. The Press will ask hostile questions regarding his right-wing record and his war-mongering statements on Iraq and Iran. The Democrats will cherry-pick and broadcast all his anti-labor comments made during the amnesty debate. For the first time, his record will be scrutinized and any previous weird/extreme/outrageous past comments will be brought up.
And how will McCain respond? Answer: badly. He is not used to being attacked. His seat in Arizona is safely Republican. The MSM has protected him. All the attacks come from the right. So when he gets angry and defensive and lashes out, it doesn't cost him - it was only the "right-wing nuts". But when the attacks start coming from the MSM and the left, his angry responses will hurt him big time. Then when he's scolded for "going over the line" by the NY times Op-ed page, he will apologize, compounding the damage. Alternating between his need for elite approval and anger at the attacks he'll be helpless against the Democrats smears and attack ads. And if Hillary is nominated the Clinton attack machine will eat him alive.