WASHINGTON — If Sens. John McCain and Barack Obama think their ads blasting each other are persuading undecided voters, they’re probably wrong. But negative ads do have an effect, an AP-Knowledge Networks poll suggests, even if it’s just to neutralize the other guy’s attacks.

They can also solidify support — or simply turn voters off to both candidates.

In a new survey, voters were asked to watch two of the presidential candidates’ negative ads, an Obama spot that says McCain would tax health benefits and a McCain ad that claims Obama wants “massive government.” The campaigns have spent millions of dollars on such ads with millions more committed for the last two weeks before Election Day.

On the whole, adwatchers who went into the experiment undecided were unmoved. About 60 percent of so-called “persuadable” voters said the ads made them no more or less likely to vote for McCain or Obama. And about a third said they were less likely to vote for either candidate after watching the ads.

Are this year’s ads fair?

More than half the voters polled believe presidential campaign commercials have been unfair or somewhat unfair. And the more ads they said they had watched, the less fair they found them. People who had seen 10 or fewer ads mostly thought they were fair, 62 percent. But people who had seen 30 or more in the past week said the opposite — 63 percent said most of the ads were unfair.

People who had seen ads by both candidates tended to think Obama’s ads were more fair than McCain’s, 39 percent to 16 percent. That may have helped Obama neutralize McCain’s critical ads.

Among partisans, 39 percent of strong Obama supporters said the ads made them more likely to vote for him, while 29 percent of McCain’s strong supporters said the same for him.

As for poll respondents’ views about what they see on TV, about four of 10 said Obama’s ads mostly attack, while about seven of 10 said that of McCain’s.

Is negative working?

n Most undecided voters, 60 percent, said the two negative ads they watched for a survey did not persuade them to vote for either Sens. John McCain or Barack Obama.

n About a third said the ads made them less likely to vote for either candidate.

AP-Knowledge Networks poll