because they think John McCain doesn't have a chance will also hurt the GOP from top to bottom--by that I mean the re-election chances of those republicans who are also running in competitive races in the Senate and the House. A depressed turnout for the Republican because of the weak showing of McCain-Palin will probably mean 60 seats in the Senate for Dems.
UPDATE: David Frum (10/26/08) : "There are many ways to lose a presidential election. John McCain is losing in a way that threatens to take the entire Republican Party down with him."
Read this too from K-Lo.