ATLANTA (AP) - Newt Gingrich is urging supporters and staff to refrain from attacks on his Republican presidential rivals.
Gingrich's campaign released a letter Tuesday in which he pledges to run a positive campaign free of negative advertising. But the former House speaker also said he reserves the right to respond when his record has been distorted
As an example, he noted a "frank exchange" with rival Mitt Romney at last weekend's debate, when the two sparred over their private sector records.
Gingrich asks supporters in the letter not to contribute to any super PAC that runs negative ads against any other Republican contender.
He said it's critical that the GOP nominee emerge from the primary "unbloodied" and able to make the case against President Obama from a position of strength.
Meanwhile, imploring supporters to stick with him, President Barack Obama acknowledged Tuesday that his re-election is "not a slam dunk" - despite his administration's achievements - because of understandable public skepticism over the economy.
Addressing donors at a hotel near the White House, the president drew attention to his efforts to heal the economy, save the auto industry, end the Iraq war and overhaul health care.
But he said: "All those things don't mean that much to somebody who's still out of work right now. Or whose house is still underwater by $100,000."
Obama said his campaign will have to fight to take its message to voters.
"This is going to be tough," he said.
Obama spoke hours after his top campaign advisers said they are uncertain about which Republican will emerge to challenge him next year but predicted a long GOP primary contest that they say will produce a weaker opponent in 2012.
Democrats have been targeting former Massachusetts Gov. Mitt Romney as the Republican most likely to challenge Obama but now say former House Speaker Newt Gingrich's surge in the polls has made the GOP contest very unpredictable.
Obama campaign strategist David Axelrod said in a briefing in Washington for reporters that he was unsure "what kind of candidate will be in the general election." He said he anticipated a lengthy primary contest that would eventually hurt the party's nominee.
Of the Republican candidates, Axelrod said: "They're being tugged to the right every day. I think they're mortgaging themselves for the general by tacking as far as they are." He said that would make it more difficult for the nominee "to scramble back" to the center and appeal to a broader base of the electorate for the November general election.
Romney and Gingrich remain locked in a close contest in early Republican voting states like Iowa, New Hampshire and South Carolina with less than a month before voters begin assessing the GOP field.