"Stab in the back" (since we seem to be dissecting metaphor on this thread) means to say one thing in support, then go back on it.
Please give me an example where McCain has failed to do what he said he would do.
How do you feel about the McCain/Kennedy immigration bill?
You are nitpicking the term, my sense is his not being a team player unless he is the beneficiary.
His CFR bill is a disaster. His lukewarm endorsement of Bush while playing nice with Kerry. His comments about the Religious Right. And the gang of 13. His support of The Wicked Witch of NY. How many do you need? He doesn't think he needs the base.
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