“This is the opening three minutes of the Super Bowl,” he told reporters after a town-hall meeting here. “We’re learning a lot about what our opponents will do. They are nastier and more dishonest than I expected, so we’ll have to make some adjustments.”

To his audience gathered inside the Atlantic Bottling Company’s warehouse, he said the results of Tuesday’s caucuses could be “very interesting.” He challenged Iowans to give him a victory to send a message that negative ads don’t work…

Nevertheless, the attacks have hurt. And asked if he’ll respond by doing the same, he said, “We may go to a much more clear contrast, but we’re not going to respond in kind. Those ads are dishonest, and [Mitt Romney] knows it. They are factually false, and he knows it. We’re not doing anything like that.”