* Ohio becomes testing ground for political science
* How many ads is too many, before viewers tune them out?
* A fine line between influencing and annoying viewers
COLUMBUS, Ohio, Nov 1 (Reuters) - It's 6:10 p.m. on a Thursday in October, just days before the U.S. elections. Before the clock hits 6:29 p.m., 11 political ads will have aired on the local NBC channel in Columbus, Ohio.
One tells voters that Democratic President Barack Obama has not proposed a legitimate economic plan for the country. Another suggests that policies of Republican candidate Mitt Romney would undermine the future for America's children.
Yet another says Romney would effectively deny many women crucial cancer screenings by proposing cuts to Planned Parenthood. The very next ad calls Obama an extremist on abortion who supports leaving babies "out to die."
Ohio is being inundated with such dueling ads in the final days before Tuesday's presidential election, as Obama and Romney both look to the state's 18 electoral votes as a crucial step toward the 270 electoral votes needed to win the White House.
The presidential race is now a fight in eight or so politically divided "swing" states, but nowhere more so than Ohio. Amid the chaos of the campaign's closing days, the state has become an arena for credibility-stretching banter, and a testing center for the growing science of political advertising.
New research predicts that total spending in this election could reach $6 billion, making it the most expensive in U.S. history. The campaigns and free-spending independent groups have poured close to $1 billion into political ads - many of those ads directed at Ohio - and have given analysts a high-profile chance to examine some simmering questions about such ads.
Among them: How many ads is too many, before viewers tune them out? And what do campaign ads lead voters to do, exactly?
Election-year political ads are a meticulously studied subject, and increasingly are used to target specific groups and encourage specific outcomes.
Some research, for example, suggests that pro-Democrat ads are particularly effective at swaying voters' opinions, while pro-Republican ads typically are more effective at getting party supporters to show up at the polls.
For all the analysis that has been done on campaign ads, academic and commercial research has yielded few answers on the precise impact that ads have in determining who wins an election.
That is especially true, analysts said, in the type of advertising free-for-all that Ohio residents are seeing on their televisions now - wave after wave of ads with overlapping and similarly dark, daunting messages.
Campaign ads became tiresome long ago for many Ohio residents, but some viewers figure that the ads must be working, or the campaigns wouldn't keep running them.
"I think poorly of those ads and don't think they work, but there are so many of them I think it must be not so," said JoAnne Harvey, a Columbus small business owner who, as an undecided female voter, is much coveted by both campaigns.
In a reflection of how so many ads can essentially nullify one another, Harvey and another dozen Ohioans interviewed generally could not recall the details of a single campaign ad that stood above the others. Those who could acknowledged that they weren't sure which side the ad was meant to benefit.
Political advertising has become a multibillion-dollar market that some television station sales managers predict soon could be a year-round category of advertising.
It has become increasingly sophisticated in "micro-targeting," the art of going after specific groups of viewers.
For example, Democrats have been found to be more frequent television watchers than Republicans, and Democrats candidates in 2008 ran more than twice as many ads as Republicans during science-fiction shows, reality dating programs and telenovelas, according to research by Washington State University professor Travis Ridout and others.
Those programs as well as talk shows and court shows tended to skew Democratic in viewership while crime and sports programs skewed Republican, Ridout's study found.
But does the science of political advertising work?
One study completed last month found Obama's ads moving voters away from Romney, while Romney's ads were much more likely to encourage Republicans to vote, rather than shift preferences among voters.
The findings were based on a survey of more than 2,300 registered voters who said they were independents or not deeply committed to one party. They were shown one or several of the campaigns' ads by the research software company Qualtrics and the research firm Evolving Strategies.
"Romney doesn't seem to have a lot of ability to have people moving in and out of the independent pool, but he has a lot of room to change the equation in determining who turns out to vote," said Adam Schaeffer of Evolving Strategies.
'THEY CAN'T STOP NOW'
If the targets of this year's ads are any guide, the presidential election will be decided by middle-aged and older white women, according to a survey of more than 1,000 buying agencies done by STRATA, a software firm whose systems help air some $50 billion worth of ads a year.
The question is whether the barrage of ads - the vast majority of them attacking a candidate, rather than promoting one - will become so overwhelming that they provoke a backlash.
Such ads "did work on me at first, and then I became a lot more cynical and realized that a lot of it is political warfare," said Harvey, who added that she voted for Obama in 2008 but was leaning toward Romney now. "It seems almost epidemic; they can't stop now that they've started."
A rule of thumb in advertising is that an ad needs to be viewed at least three times and up to 10 to be effective, said STRATA Chief Executive John Shelton.
"There's no question that once you start to go over (10), you start to, well, at least bore people," he said. "Then they might tune out. Then they might actually get ticked off."
Barbara Berry, a healthcare professional and Obama supporter from Columbus, said she pre-records TV programs and skips ads.
"I don't pay attention anymore," she said.
Since late August, more than 915,000 presidential campaign ads have aired on broadcast and national cable TV, according to the Wesleyan Media Project. In Columbus during October, ads by the campaigns and outside groups aired more than 7,000 times.
"Some of them just disappear in the noise," said Dan Bradley, general manager at the Columbus NBC affiliate WCMH-TV.
Each presidential campaign has been producing about a dozen new ads a week, basing them on daily news events - a practice that ensures that most of the ads have a short shelf life.
Romney in particular tends to place and replace ads at the spur of the moment, often in response to the news of the day.
Obama's campaign runs two ad tracks: one that changes every one or two days, the other every couple of weeks.
Ads this year "just seem to be rushed," said John Geer, a political ad researcher at Vanderbilt University. It's "almost like they've fallen prey to the fact that the campaigns have so much money, and the ability to make all these ads."