Greg Sargent has a post, “Millions in TV ads, but no poll movement for Scott Walker” up at WaPo’s ‘The Plumline’ noting an interesting phenomenon.
Citing a new Marquette Law School poll indicating that Wisconsin Governor Scott Walker is locked in a dead heat with Democratic challenger Tom Barrett among WI RV’s, Sargent explains:

…Walker’s approval rating, and his head to head numbers with Barrett, have not changed in months — if anything, they’re going down. And this is in spite of the fact that Walker and his allies have vastly outspent rivals in TV ads.
Charles Franklin, a political scientist and expert in Wisconsin politics who directs the Marquette poll, sends over some numbers…In January, Walker’s job approval was 51 percent; in March, it was 50 percent; and this month, it’s 47 percent…In January, Walker was leading Barrett 50-44; in March, 47-45; and this month, he trails 46-47. (Among likely voters, Walker leads by a point; all of these findings suggest a mostly unchanging dead heat.)
“There’s been a great deal of advertising in the state, especially from the Walker campaign and Republican supporters, and we’ve seen virtually no movement in the Walker numbers,” Franklin tells me.

Sargent adds that Walker has just announced that “he’d raised a staggering $13 million in three months for the recall fight,” but still no improvement in the polls for Walker. Sargent quotes Franklin again: “This means the race is all going to come down to turnout — the one area where Dems and unions can match Walker in resources and organization, perhaps neutralizing Walker’s ad spending advantage…The advantages that Democrats and unions have traditionally had in the ground game is certainly an area where they can match Walker’s organization at the very least.”
It may be that the same dynamic is at work in the presidential campaign — that there comes a point at which each additional dollar spent on campaign ads brings diminishing returns. In the home stretch, however, it’s all about the turnout ground game — and that bodes well for Democrats.