Depends what you mean by Conservatives. Not all Rep are Conservatives in the sense I believe you mean. I am a Republican and don't believe I fit the mold of many here on FR. I am totally against abortion, support gun owners, love to death tax cuts, want our borders secure and am a literal fanatic about national defense and winning the WOT. I also want smaller government that spends less BUT I also believe that these things are not all of the same importance all the time.
Today the national security issue including the overseas WOT and the proper surveillance techniques here is of primary importance. It is in this venue I believe McCain is head and shoulders above anyone on the left and most especially the empty suit.
That said, what ever hard line Conservatives McCain loses (and I don't think in the end it will be near as many as you think, he will more then make up for by grabs from the center and right.
Who do you think will make more screwups over the next few months McCain or Obama? I think McCain will continue to crap all over the conservative base. But Obama seems destined to have some John Kerry moments. Then again, I thought the oil bubble would burst by September. Doesn't appear that this will happen. If it doesn't Obama can make all the idiotic statements he likes and he'll beat McCain.