Because a large part of McCain's appeal is with independent
voters, something the war has already eroded. McCain's best
performances in 2000 -- indeed, almost all his wins -- came in open
primaries. Other than Arizona, he lost actual Republicans in every
state. That even includes liberal states like Massachusetts, where
he won over 60 percent because of huge margins among
independents.
Now, McCain ended up losing the nomination for the very same
reason -- Bush cleaned his clock when the schedule shifted to
closed primiaries. So perhaps McCain's war stance will help him
attract more Republican base support. But it is definitely a
gamble, because it is based on trying to win over McCain skeptics
at the expense of possibly alienating his strongest supporters.