JACKSON, Miss. — Republican Gov. Haley Barbour bowed out of presidential contention Monday with a surprise announcement just as the 2012 campaign was getting under way in earnest, 18 months before Election Day. The Mississippi governor said he lacked the necessary “absolute fire in the belly” to run.

Barbour’s declaration, unexpected because he had been laying the groundwork for a campaign for months, thins a Republican cluster of a dozen potential candidates to take on President Obama.

With the GOP campaign’s first debate scheduled for next week, the muddy Republican field will become clearer soon as more potential contenders announce whether they’ll run or sit out. The next, facing a self-imposed deadline of this weekend, is Indiana Gov. Mitch Daniels, a Barbour friend and a fiscal conservative who has shined a spotlight on rising budget deficits and national debt.

“All eyes will be on Daniels. … It’s a clear path for him if he wants to run,” said Doug Gross, an Iowa Republican.

“I will not be a candidate for president next year,” Barbour said a statement, adding that he wasn’t ready for a “10-year commitment to an all-consuming effort.”

As the GOP race comes into sharper focus, Obama is working to both prevent an erosion of his support while under Republican attack and to raise enough money to overwhelm his eventual foe. He’s been packing his schedule with fundraisers and visits to battleground states as he gears up for what he says will be a tough campaign.

This week alone, he will raise money in New York and return to his hometown of Chicago to tape an episode of “The Oprah Winfrey Show.” He then will head to Florida, a pivotal swing state, to deliver a commencement address at Miami Dade College and attend the launch of Endeavour, NASA’s next-to-last space shuttle flight.

Potentially vulnerable, Obama has middling poll ratings and is seeking a second term in a country reeling over high unemployment, rising gas prices and the remnants of a recession.Yet, the GOP faces plenty of its own troubles.

Its field lacks a front-runner. Most of the candidates are largely unknown to Republicans. The most recent Associated Press-GfK poll indicated that only half of all Republicans were satisfied by their choices and a third were dissatisfied.