your voice, your vote. Something new in the brand new presidential race tonight. For months, president obama and mitt romney have been locked in a dead heat. This evening, within candidate appears to... See More
your voice, your vote. Something new in the brand new presidential race tonight. For months, president obama and mitt romney have been locked in a dead heat. This evening, within candidate appears to have pulled ahead. The threw new major polls tonight. You can see, cnn, reuters, has the president ahead. What's behind the latest poll numbers? Let's go to jon karl on the campaign trail this evening. Reporter: As romney prepares to make his biggest decision yet several of the polls now show he is losing to president obama, and by a significant margin. The numbers suggest a relentlessly negative assault by the obama campaign has taken its toll. The latest salvo, an ad featuring the answer rom flee gave during an interview with our own david muir. Was there ever any year when you paid lower than the 13.9%? I haven't calculated that. I'm happy to go back and look. Did romney pay 10% in taxes? 5%? 0%? We don't know. Reporter: The ad comes after that controversial charge from senate democratic leader harry reid that romney went ten years without paying any taxes. He said he had a secret source, but won't reveal it. Does the president think this allegation made on the senate floor, is that below the belt? Does that cross the line? Again, I have not -- the president has not expressed an opinion to me on this. Reporter: The tax return ad comes just after the controversial ad by a pro-obama group that suggested that romney was responsible for the death of a man's wife from cancer. Romney campaign fired back today with a response ad. Doesn't america deserve better than a president who will say or do anything to stay in power? Reporter: Next three weeks may be the most important of romney's campaign. Starting tomorrow, launching a bus tour of virginia, north carolina, florida and ohio, all states obama won in 2008. Coming any day, his vice president shl choice. And starting just 17 days from now, the republican convention. The romney campaign says that widd up to one big bounce. And a chance, david, to finally pull out ahead of president obama. All right, jon karl, can't wait to join you there on the trail. In the meantime, sunday morning on "this week," george dives into the new numbers with tim pawlennd president obama's chief political strategist, david axelrod, all facing the tough questions sunday morning with george.
This transcript has been automatically generated and may not be 100% accurate.