?All the factors say go but my heart says no, and that?s the decision that I have made,? he said near the end of his Fox News program, ?Huckabee.?

Mr. Huckabee made clear in recent weeks that he was seriously considering waging a campaign after sending earlier signals he was not. Although Mr. Huckabee had said he would announce his intentions on the show, even his closest aides did not know what he would say.

He kept his supporters waiting as long as possible, making the announcement at the very end of his hourlong, 8 p.m. program after interviewing guests including the ?Extra? host Mario Lopez and the rock musician Ted Nugent, even jamming with him on his old hit song ?Cat Scratch Fever.? (Mr. Huckabee plays the bass.)

Even during his statement, he used language that seemed to hint he was running. ?The past few weeks the external signs and signals and answers to many of the obstacles point strongly toward running,? he said. But, he said, upon reflection, he realized, ?under the best of circumstances being president is a job that takes one to the limit of his or her human capacity. For me to do it apart from an inner confidence that I was undertaking it without God?s full blessing is simply unthinkable.?

Mr. Huckabee had been showing strong poll numbers, among some of the strongest of his party?s possible contenders. His decision brings the fluid field of prospective Republican presidential candidates a step closer to a solid state, leaving two other highly anticipated announcements to come: those of Gov. Mitch Daniels of Indiana and former Gov. Sarah Palin of Alaska.

Former Gov. Jon M. Huntsman Jr., of Utah, who recently left his post as President Obama?s ambassador to China, is seriously considering a run, as is a former ambassador to the United Nations, John R. Bolton, who said in a brief interview Saturday night that Mr. Huckabee?s announcement made him more likely to start a campaign. ?Anyone else considering it has to get serious about it,? he said.

(An aide to Representative Michele Bachmann of Minnesota said on Friday that she would be more likely to run, as well, if Mr. Huckabee decided against doing so.)

Had Mr. Huckabee entered the race, he would have been filling a perceptible void in the Republican field by becoming the presumed candidate of evangelicals, whose social issues have been largely swept aside as both parties focus most acutely on the economy and government spending.

Mr. Huckabee, an ordained Baptist minister and veteran of religious broadcasting, has carefully maintained ties to his colleagues in the religious sphere.

?He knows our issues backward and forward, and he knows our people backward and forward because he is one of them,? said Dr. Richard D. Land, the president of the Ethics & Religious Liberty Commission of the Southern Baptist Convention.

His decision to forgo a run presumably leaves that space wide open for Ms. Palin, a self described ?Bible-believing Christian,? should she jump into the race. But Mr. Land said there were candidates already running who can and will address the critical evangelical constituency, among them former House Speaker Newt Gingrich ? who has been actively reaching out to Christian groups ? and former Senator Rick Santorum of Pennsylvania, who has been outspoken against same sex marriage and on in other conservative social issues causes.

Still, evangelicals have yet to rally to the side of any of the candidates thus far. In that regard, they resemble the party in general. Said Matthew Dowd, a former political adviser to President George W. Bush: ?It?s anyone?s game at this point.?