Charles Dharapak/AP Photo

ABC News' Jonathan Karl and Elicia Dover report:

Mr. Positive is about to go negative in a big way. Look for Newt Gingrich to unload on Mitt Romney after the Iowa caucuses.

Gingrich won't called it negative. He'll call it telling the truth. But sources familiar with his plans say he plans to hammer Romney hard, especially in South Carolina.

He's hinted at the change over the past day or so. He told ABC News Friday he plans to make "adjustments" to how responds to the negative he's taken.

Today he told reporters Iowa is an "wonderful experiment" on what happens to a candidate who stays positive in the face of some attacks.

"The average Iowan is watching the tsunami of negative attack ads and they've watched one guy be consistently relentlessly positive. Whatever the outcome of Tuesday night, we'll learn a lot."

He sees he's already learned something about his opponents.

"We're learning a lot about what our opponents will do," Gingrich said. "They are nastier and more dishonest than I expected. So we'll have to make some adjustments."

Look for one big adjustment: a feistier Newt Gingrich ready to tear into his opponents - especially a certain former governor of Massachusetts.