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And I'm Renee Montagne.

Here's the most prominent fact about all the presidential campaign ads flooding nine battleground states. Most of them don't urge you directly to support their candidate. Instead, each campaign wants you to be very, very worried about the other guy.

The campaigns are on track to spend one billion dollars on advertising alone, and NPR's national political correspondent Mara Liasson reports on what they get for the money.

MARA LIASSON, BYLINE: If you live in Virginia, Nevada, Iowa, Ohio, New Hampshire, Colorado, Florida, North Carolina or Wisconsin, unless you live in a cave, you cannot get away from this.


UNIDENTIFIED WOMAN #1: In 2008, I voted for Barack Obama. He doesn't have my vote this time...

UNIDENTIFIED MAN #1: They would be throwing all that away in the trash can if they voted for Mitt Romney.

UNIDENTIFIED MAN #2: Obama's second term would be a rerun of the first and our country just couldn't survive that.

UNIDENTIFIED WOMAN #2: No matter what Mitt Romney's ads say, we know what he'll do.

LIASSON: To the folks who track political advertising at Kantar Media CMAG, these commercials tell a story. First, they're overwhelmingly negative by almost seven to one, and the chessboard is set. With the addition of Wisconsin in June, the battleground of states where ads are bought hasn't changed, and because both sides have so much money, even if some states are beginning to look out of reach - say, North Carolina for the Democrats - the ad battleground will not shrink this year, says Ken Goldstein, the president of Kantar.

KEN GOLDSTEIN: So it used to be, you'd sit around and go, we only have a finite amount of money because we've taken federal funds. And those campaigns used to have to make very, very difficult decisions, triage decisions, about where they were going to play and where they were going to leave. You really don't need to have those tough decisions now.

LIASSON: But with all that money, the campaigns could be expanding the battleground, but the ad data tells us they're not. Republicans, for example, are not yet advertising in Michigan or Pennsylvania.

As for who has the advantage in the ad wars, Goldstein says until recently the Obama campaign was able to put more lead on the target.

GOLDSTEIN: Obama and the Democrats, actually for much of late August and almost all of September, had an advantage in message flow in terms of paid advertising. That's been equal the last couple weeks.

Everyone believes we're going to see a huge amount of Republican money the last two weeks, but the last time we looked at these data - five minutes ago - both sides are up at very equal levels in all the markets that matter.

LIASSON: The Obama campaign had an advantage because superPACs and parties pay more per spot than campaigns, which get a discount. And if you book your ad time early as the Obama campaign did - as opposed to at the last minute, the way the Romney campaign does - you can get a cheaper rate.

And no matter who you are, says Goldstein, if you want to make sure your ad reaches a target group of persuadable voters in the right states, you pay a lot more.

GOLDSTEIN: What may be the most valuable is college football in the Midwest. If white males are a target, what do white males like to do? Like to watch football, and like to watch lots of football. That becomes a very, very valuable, valuable buy.

LIASSON: But do ads actually work? Political scientist Diana Mutz is skeptical.

DIANA MUTZ: There's very little evidence that ads make much of a difference in a presidential campaign. Most people are shocked when they learn about what the likely effects are relative to the huge amount of campaign resources that gets poured into advertising. It's shockingly small bang for the buck.

LIASSON: But this year the campaigns are happy to settle for shockingly small. Goldstein figures that there are about 800,000 truly undecided voters in the battleground states. Factor in a total of $1 billion in advertising and that means campaigns are spending about $1,000 per persuadable voter.

GOLDSTEIN: And so you have tremendous numbers of ads chasing very, very few eyes. Now, that said, you may say, well, that's not very efficient. But these campaigns aren't worrying about efficiency. These campaigns are worrying about Al Gore. All of this stuff matters at the margin. Ask Al Gore if the margin matters.

LIASSON: Al Gore, of course, lost the 2000 election by 537 votes in Florida, a point the Obama campaign happens to be making in a new ad. Goldstein also thinks ads might explain why Romney is doing better in the national polls than in the battleground, where voters have been exposed to a barrage of anti-Romney ads.

Look at Ohio and Wisconsin. Ohio was expected to be close and the Obama side pummeled Romney with negative ads all summer. The president retains a small but measurable advantage there.

In Wisconsin, the Obama campaign was thought to have an advantage, so Democrats didn't start advertising until this fall, leaving the field to the GOP. Romney is now doing better in Wisconsin than he is in Ohio.

And what about saturation? Are voters tuning the ads out? Lynn Vavreck is another political scientist who studies ads and she says there's no point at which voters tune out political commercials.

LYNN VAVRECK: The ads have effects but those effects decay pretty rapidly. So if you're the Obama or the Romney campaign, one of the things you need to do is be consistently on the air. You cannot cede any part of the game to your opponent, because then those effects will start to accumulate.

LIASSON: So long as there is one more voter out there to be persuaded, the ad wars to get them will continue.


UNIDENTIFIED WOMAN #3: ...responsible for the content...

LIASSON: Mara Liasson, NPR News, Washington.




