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Maybe, just maybe, a series of positive ads might make the difference on Election Day in a key race for the United States Senate.

To be sure, Iowans weighing whether to vote for Republican Joni Ernst or Democrat Rep. Bruce Braley have seen their fair share of negative ads, which remain a mainstay of just about every tight race.

Yet there are signs that Ms. Ernst鈥檚 attempts to cast herself in a positive light be having a measurably positive effect in what is a neck-and neck race. TV and radio advertising have associated her with motherhood, leadership as an Army National Guard officer, and the values learned working on a farm. The ads could be a factor in her rising name recognition and favorability ratings, political analysts say.

Indeed, the Iowa race shows how positive ads work best, analysts add. They help candidates who are not incumbents define themselves, and if the race turns very negative 鈥� as many races have 鈥� they can give voters a positive message to latch onto or effectively defuse attacks.

In Iowa, Representative Braley has failed to come up with much positive messaging of his own.

鈥淏raley failed to define himself,鈥� Republican pollster Neil Newhouse said at a recent Washington event analyzing the midterm congressional races. Mr. Newhouse recounted how a focus group of working-class Iowa women 鈥渒new a lot about Joni Ernst, positive and negative, [but] nothing about Braley.鈥�

Positive ads have long been outnumbered by negative or 鈥渃ontrast鈥� ads in US election campaigns. But in the Sept. 26 to Oct. 9 period this year, the number of positive ads actually rose in Senate races (to 29 percent of the total) compared with the same period in 2010 (25 percent) and 2012 (20 percent), according to Kantar Media numbers released this week by the Wesleyan Media Project in Middletown, Conn.

In one positive ad (see top), Ernst takes to the camera to tell voters 鈥渨hat I really care about,鈥� mentioning things like Social Security and schools 鈥� which happen to be topics that Democrats have attacked her on.

In another ad (see right), military colleagues chime in with praise for her leadership.

鈥淲e were getting ready to deploy to Iraq,鈥� a vet who served with Ernst says in one campaign ad. 鈥淛oni said the most important thing is to bring everybody back alive. That鈥檚 when I knew that Joni was a commander, a real commander, someone who cared about soldiers.鈥�

The gauzy ads haven鈥檛 made Ernst the commander in a race for the Senate seat vacated by retiring Democrat Tom Harkin. Recent polls have her leading but only by a few percentage points on average.

But nearly 43 percent of Iowa voters have a 鈥渇avorable鈥� view of her, by one recent poll. That 鈥渇avorable鈥� number has risen during the past month, in polling by Loras College in Dubuque, Iowa, even amid an onslaught of negative ads. And it comes as Braley hasn鈥檛 budged his favorable rating beyond 37 percent.

Candidates typically run a mix of ads, and several factors may explain why the negative ones dominate.

Foremost is simply that attacks are often effective.

鈥淭here's a lot if evidence to suggest that negativity works鈥� in influencing voters, says Erika Franklin Fowler, a political scientist at Wesleyan University and a co-director of the media project.

Although voters say they prefer positive ads, criticism 鈥渃an provide the substantive information that citizens need,鈥� Ms. Franklin Fowler says. Positive ads are often little more than vague paeans to ideals like freedom and family.

Negative ads are also favored by outside political groups, which have grown in prominence thanks to court rulings affirming their right to spend unlimited amounts as long as they鈥檙e not officially connected to candidates or political parties.

Still, when the overall tone of campaigning is dark, the attacks may take on a 鈥渟ame old鈥� ring for voters. Ads with an affirmative message may gain in significance.

鈥淚f there's anything to be said for the strategy of going positive, it's that the positivity may stand out against the backdrop of negativity,鈥� Franklin Fowler says.

In open-seat races like Iowa, where the candidates were unknown by many voters, ads that say 鈥渉ere鈥檚 who I am鈥� can be especially important.

But positive ads can also be an asset for incumbents faced with tough challengers.

In New Hampshire, Sen. Jeanne Shaheen (D) has used ads to remind voters of tangible ways her efforts have touched the state.

And on the Republican side, an ad rolled out by Senate minority leader Mitch McConnell casts a human glow on a man most often seen waging partisan battles against his nemesis, majority leader Harry Reid.

In the ad (see right), a Kentucky woman recounts how Senator McConnell helped when her daughter was abducted and taken to Mali by her former husband. 鈥淚 didn鈥檛 know whether she was alive or dead,鈥� Noelle Hunter says. She says 鈥渉e took up my cause personally,鈥� pushing the State Department and the government of Mali for a solution, and then meeting Ms. Hunter at the airport when she was reunited with her daughter.

It鈥檚 a feel-good ad, but checking up on Ms. Hunter found a complicated back story for her, including drug abuse, that was merely hinted at by the McConnell campaign. (The ad quotes Hunter saying the senator鈥檚 help came 鈥渁fter a dark period in my life.鈥�)

That鈥檚 a cautionary reminder: It鈥檚 risky to take political ads 鈥� even the positive ones 鈥� at face value.