Isn't this a mismatched headline? I expected there to be a poll showing that Feingold has, in fact, boosted his standing among voters. That the idea is popular among Democrats doesn't mean that Feingold has gained support among voters generally (or even, for that matter, that Dems are more likely to support him.)
My guess is that Feingold has garnered some attention and support for himself on the left side of his party, but he has done so with a stunt that will be just another way to call him weak on national security. This double-edged effect is why conservatives call his ploy a gift and he calls on the right to "bring it." Both sides are getting what they want.
But being weak in security issues is the largest single reason that Dems lose the Presidency. Good thing they remain in denial about that.