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INDEPENDENCE, Iowa - No more Mr. Nice Guy.  Newt Gingrich will "draw a very clear contrast" with Romney "every day" immediately after the Iowa caucuses Tuesday, the former House speaker told ABC News.

"Everything we say will have Romney's quote, Romney's videotape, Romney's record; it'll all be based explicitly on Romney," Gingrich said in an interview in Independence, Iowa.

Gingrich has said repeatedly that Republicans should aim their attacks at President Obama, not fellow Republicans.  But after getting hammered by millions of dollars in negative ads, Gingrich says he will now return fire, targeting Romney over and over again.

"He set the tone of the campaign," Gingrich said.  "By going after me negatively and dishonestly."

But won't his plan violate Gingrich's promise to run a positive campaign?

"Well, if the facts are negative, then that tells you about his career," Gingrich said.

Even if Romney wins Iowa and New Hampshire, Gingrich said he cannot win the Republican nomination.

"Romney is, in the end, in a culturally hopeless position," Gingrich said.  "He's a Massachusetts moderate in a conservative party."

Asked directly whether Romney can win the nomination, Gingrich said, "No. The minute people understand his record, it becomes not him."

"If he wins here, he's going to win with what, 20 percent of the vote, 22 percent. That means he'll have spent about 400 per vote," Gingrich said.  "That means, 77 percent of the votes will be not Romney."

And while Gingrich says he has given Romney a free pass so far, he plans a new, aggressive effort to "draw a very clear contrast with Romney every day."

See ABC's Interview with Newt Gingrich.

Earlier in the day, Gingrich flatly admitted he cannot win the Iowa caucuses and put the blame on millions of dollars in negative ads run against him here.

"I don't think I'm going to win," Gingrich told reporters.  "But, on the other hand, if the Des Moines Register is right that it's 41 percent potentially undecided, who knows what's going to happen."

In the ABC News interview, Gingrich also predicted a Romney victory in New Hampshire.

"I'm sure Romney will win" in New Hampshire, Gingrich said. "But we need to bring his margin down if we can. And we need to lay the case. In New Hampshire, you're also laying the case for South Carolina and Florida and beyond."

See more of from Iowa and ABC on World News with Diane Sawyer.