I don't think a blowout is unrealistic.
Caution, though: there's w-a-a-a-y too much time left in this campaign, and anything can happen.
But provided John McCain keeps his nose clean and gets the Hell busy paying attention to Republicans who aren't comfortable with him, I think it's realistic, a blowout.
That however is a pretty chancey "provided."
As for "coat-tails," I'm not sure, I don't think so.
This is why I'm, personally, worrying less and less about the presidential campaign, and more and more about the congressional campaigns, although my feelings are that Republicans won't do too badly. Not great, but not badly, either.