Washington ? 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.

The campaign officials said the president’s speech last week in Kansas offered a glimpse of what his message will be next year: His argument that the middle class has faced numerous challenges during the past decade and that the country’s economic policies must give everyone a “fair shot and a fair share.”

Obama made that case again in his remarks to donors, telling them “we’re all in this together.”

“That vision can contrast to a vision that basically says you are on your own,” he said. “It’s what this election was about in 2008; it’s what this election is going to be about in 2012.”

The campaign officials also claimed an organizational advantage over the GOP. They said they have more staffers on the ground in Iowa than the Republicans and have had about 1 million conversations with supporters and about 90,000 in-person meetings with volunteers since Obama launched his re-election campaign in April.

Obama’s campaign outlined several potential paths to victory that would build upon states that Democrat John Kerry won in 2004 and winning in Western states like Colorado, New Mexico, Nevada or holding onto Southern states Obama captured in 2008, such as Virginia and North Carolina.

Obama’s session with top campaign donors came ahead of the Dec. 31 deadline for the current fundraising quarter. Obama has raised more than $150 million for his campaign and the Democratic National Committee through the end of September.