I doubt all the 18% who said Paul, would vote Paul. I suspect its a protest vote in this poll that has pushed him so high.
Correct answer. Once the GOP consolidates around its nominee (presumably Romney) that 18% would drop. As the campaign nears its end the number would drop further, just like any third party candidate, as the appeal of a protest vote fades in favor of worries the greatest evil among the three candidates (for most Paul voters, Obama) will win.
After an initial candidacy announcement bounce, Paul'd have trouble maintaining the 15% threshold for debate invitations. He wouldn't win any states, and MIGHT finish second in one or two (or ME-2).
At that point, Paul supporters would be showing other Republicans why they lost (even if Paul only has 5-8%). That's why so many Paul supporters want Paul to run in the general, no matter what. They want the other Republicans to know that they refuse to support RINOs. There is no significant difference seen between Romney, Cain, Gingrich, or Obama.
