I assume this attitude stems from the belief that whoever wins the nomination is irrelevant, as all Republicans are, at heart, fiendish beasts of the far right and to be shunned. This is perfectly true. Unfortunately, it is also true that the American political landscape is skewed so far to the right that even a McCain - who would count as an extremist in any sane coutry - matters.
The Republican Party was not always the monster it is today. It was the party of Thomas Jefferson and Abraham Lincoln. It was also the party of Dwight Eisenhower, who opposed segregation, expanded social security, espoused "Dynamic Conservatism," condemned the bombing of Hiroshima and Nagasaki, and warned of the encroaching power of the "military-industrial complex" (2). It was, in fact, once a party of principle.
Somewhere along the line, of course, it became snarled up into an ugly mess. The Christian morality that underlay it became twisted into fundamentalism, and the party embraced the foaming madness of the anti-abortion lunatics. Inherent suspicion of government became perverted into the carpet-bagging of Reaganism and neo-conservatism. Forget keeping a wary eye on the possible excesses of the state - simply privatise, sell off, cut.
The weird alliance of the neo-cons and the religious right has maintained its control over the party since the late 70s. It has excluded the rump of the Republican party - people who believe in small governemnt, low taxes, values and morality, but are not comfortable in the extreme positions of the ideologues who have colonised their party.
We've seen these ideologues turning on McCain (4). This is because they fear he will challenge their control of the party. They might even be willing to lose an election to stop him and to crush the caucus he represents.
He isn't - by any sane definition - a liberal. Rank and file Republicans recognise this, not matter what the likes of Coulter and Limbaugh may say. But he is a maverick who likes to challenge authority. One you get to be President of the United States, of course, there isn't much authority left to challenge, except the power of the groups who control the Republican Party. He might do it just for the Hell of it. He might do it because he feels an obligation to reclaim the party for the moderate rump of the party. He might, of course, do nothing at all.
Because the USA operates a de facto two party system and politics in the USA seems to be entirely an exercise in propganda, not fact, the Republicans and the Democrats are linked. The lurch to the right by the Republicans dragged the Democrats along with it - the party of Clinton (Mark I or II) isn't the party of Roosevelt or Kennedy. But if McCain can weaken the grip of the religious and economic extremists, then might allow the Democrats room to re-position as well. An America where the choice is between moderately sane and moderately insane is an improvement on an America where the choice is between the moderately insane and the utterly moonstruck.
It's tempting to write off any Republican candidate as just apuppet of those forces, but that's simplistic. The party is as prone to schisms, civil wars and calamity as any other human organisation. McCain's popularity is a symptom of the struggle between the people who control the Republican Party, and the people who actually vote for it.
1 - 'A Draw,' posted by Idiot/Savant on Nor Right Trun, 7th of February, 2008. (http://norightturn.blogspot.com/2008/02/draw.html)
2 - Opposition to segregation, expansion of social security, espousal of "Dynamic Conservatism" as per his autobiography on Wikipedia, as viewed on the 10th of February, 2008: (http://en.wikipedia.org/wiki/Eisenhower). His opposition to the use of the atomic bomb as per wikipedia, same date (http://en.wikiquote.org/wiki/Dwight_D._Eisenhower#Post-Presidency). His farewell address contains his comments on the military-industrial complex: (http://www.americanrhetoric.com/speeches/dwightdeisenhowerfarewell.html).
Condemnation of the use of the atomic bomb in 3 - McCain's a Baptist, but only nominally. He was raised as an Episcopalian. As per his Wiki biography, viewed 10th of February, 2008. (http://en.wikipedia.org/wiki/John_McCain#U.S._Congressman_and_more_children)
4 - As described previously on lefthandpalm: http://lefthandpalm.blogspot.com/2008/02/why-republicans-hate-mccain.html and http://lefthandpalm.blogspot.com/2008/02/far-right-out-for-mccain.html.