The only other alternative is stay home.
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No, you can vote down ticket for conservatives and vote conservative (not GOP) for POTUS. Many of us are not planning on staying home and voting against BOTH Mitt and Obama.
That's true. Actually, some positive could come out of this by doing it as you say.
First of all, I don't see Obama leaving the WH even if Romney won the election. Obama will refuse to concede because "it will be the right thing to do".
And we certainly do not need a liberal republican spending the next 4 years claiming to be a conservative. That would just move center leftward. By the time Romney finished his term even Susan Collins would be considered too "right wing" to ever be President.