"It's embarrassing for the Senate. It's embarrassing for our party," said Sen. John Ensign of Nevada, who leads the GOP's Senate campaign committee.

"I think it's best for the U.S. Senate, it's best for certainly his party, that he just keeps his word," Ensign told reporters outside the Senate chamber Thursday. "He gave us his word he would do something, and he's backing out on us, and I don't think that's the right thing to do."

A judge ruled Thursday that Craig's guilty plea to a misdemeanor disorderly conduct charge could not be withdrawn. Craig entered the plea after his June arrest in a men's room at the Minneapolis-St. Paul International Airport for allegedly propositioning a plainclothes police officer for sex.

After news of his arrest and guilty plea broke in August, the senator announced that he would resign at the end of September, but he postponed that move while seeking to have his guilty plea withdrawn.

However, after the judge's ruling Thursday, Craig said he'll remain in the Senate and study "additional legal options" -- much to the consternation of some GOP colleagues. Watch how Craig's decision poses a challenge for GOP leaders »

Sen. John Thune, R-South Dakota, called the issue "a distraction" for the party.

"I would have hoped he would have done what he said he was going to do," added Sen. Norm Coleman, R-Minnesota.

"He's chosen not to. The people of Idaho will have to speak to that, and the Ethics Committee will have to speak to that," Coleman said, referring to the Senate panel's indication that it will investigate Craig's conduct.

Fellow Idahoan Sen. Mike Crapo was a rare voice in the caucus, issuing a statement in support of Craig's decision.

"I look forward to serving with him as we continue to work on issues important to Idaho," Crapo said.

But in Boise, a spokesman for Idaho Gov. C.L. "Butch" Otter said that the Republican governor already has settled on a replacement for Craig -- but won't name the choice until the senator steps down.

Spokesman Jon Hanian denied that Otter, a longtime friend, is pushing Craig to go, saying the governor "just wants to be ready."

A Craig resignation would give an appointee the advantage of being in office for a year or more before the next election. Craig said he won't seek re-election in 2008, a decision advisers said he had made earlier.

The lack of an Idaho incumbent -- if that happens -- would complicate things for the GOP in what already is shaping up to be a difficult 2008 election.

Four other Republican senators -- John Warner of Virginia, Chuck Hagel of Nebraska, Wayne Allard of Colorado and Pete Domenici of New Mexico -- have announced they will not seek re-election, and at least four others are expected to face tough races.

"Democrats are cheering today," the Idaho Statesman newspaper said in an editorial urging Craig to go ahead and resign.

"He and we will be distracted by a Senate Ethics Committee and its hearings, which could be public, in what is certain to be brutally ugly, possibly on live TV, and centered on men having sex with men," the editorial warned.

Jasper LiCalzi, a political science professor at Albertson College of Idaho, told the Idaho Statesman that some voters still back Craig.

"There is core support here that thinks Craig has been railroaded, and there's another group that thinks he should stick it out," LiCalzi told the newspaper.

In his statement Thursday, Craig said any replacement would lack the seniority and committee assignments "that are valuable to Idaho." In addition, he said he wanted to clear his name before the Senate Ethics Committee.

One of Craig's attorneys, Stanley Brand, said on NBC's "Today" show Friday that Craig is considering appealing the judge's ruling and seems willing to risk a Ethics Committee inquiry.

"What he is saying is in 220 years of recorded history ... the Senate has never disciplined anyone for misdemeanor conduct that has nothing to do with official duties," Brand said on "Today." "So, while the Senate theoretically could do that, it hasn't, and our position is there is no reason to start now after 220 years."