ATLANTA -- Newt Gingrich is pledging to stay relentlessly positive in his quest for the White House. Except when he's not.

Trying to make over his image as the angry, bomb-throwing leader of the Republican revolution of the 1990s, the former House speaker has adopted a sunnier persona these days and is playing up his credentials as a grandfather, husband and historian.

On Tuesday, he urged supporters to refrain from attacking his opponents and eschewed negative ads.

But old habits die hard.

When chief rival Mitt Romney cast Gingrich as a lifelong Washington insider at a weekend debate in Iowa, Gingrich had this snarky comeback, "The only reason you didn't become a career politician is because you lost to Teddy Kennedy in 1994."

In New Hampshire on Monday, Gingrich lashed out at Romney, calling on him to "give back all the money he's earned bankrupting companies and laying off employees" when he ran the private equity firm Bain Capital.

In Gingrich's camp there is simple explanation: Romney started it.

But Gingrich said that while he won't go negative, he won't unilaterally disarm either.

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"I have reserved the right to respond when my record has been distorted," he said Tuesday in a letter to supporters and staff that urged them not to attack his opponents. He referred to Monday's back-and-forth as "what in diplomatic circles is called 'a frank exchange' over our respective records in the private sector."

For some political veterans, the episode brings back memories of the scorched-earth tactics Gingrich was known for as he engineered the first Republican majority in the U.S. House in decades during the 1994 congressional elections on the strength of his fiery rhetoric.

"Newt creates political success by drawing the starkest possible contrast between your position and your opponent's position, even if turns out to be hyperbolic, that is acceptable under Newt's rules of engagement," said Rich Galen, a former Gingrich aide.

Bob Barr, a Gingrich supporter and former congressman from Georgia, said negative campaigning gets bashed. "But a lot of times it does work. That's why people do it."