Fox News is told McCarthy simply said it was not his time. McCarthy faced opposition from some conservative members and groups, but was thought to have more than enough support to win the party's nomination in the vote initially set for Thursday. 

It's unclear what specifically made McCarthy change his mind and drop out. 

ORIGINAL STORY ... 

House Republicans are huddling on Capitol Hill to nominate the next speaker, as Majority Leader Kevin McCarthy grapples with an 11th-hour challenge to his bid to succeed John Boehner -- after a conservative bloc threw their support elsewhere. 

McCarthy, though, voiced confidence going into the vote, which will pit him against Reps. Jason Chaffetz, R-Utah, and Daniel Webster, R-Fla. 

"It's going to go great," McCarthy said Thursday morning. 

Webster has won favor with the conservative wing of the party. Wednesday evening, the House Freedom Caucus -- with its 30-40 members -- decided it would back Webster as a bloc. 

The results are expected to be announced Thursday afternoon. 

While McCarthy remains the favorite to win Thursday's nomination, the entire House -- including Democrats -- will cast the final vote for speaker set for Oct. 29. 

And even if McCarthy wins the nomination comfortably on Thursday, he'll need to muster an absolute majority, of roughly 218 members, to win at the end of the month. If he -- or any candidate -- struggles to reach 218 votes before the end of the month, that potentially gives other factions leverage going into the vote. 

McCarthy, meanwhile, is said to be trying to distance himself from Boehner, amid conservative concerns he'd represent a mere continuation of the sitting speaker's term. The other candidates also are vowing a fresh start. 

"I think McCarthy's pitch was `I'm not John Boehner, I'm going to run things differently, I'm my own man,"' Rep. Blake Farenthold, R-Texas, said after the candidates made their pitches to members during a meeting Tuesday. 

The speaker's race already has seen a few curveballs since Boehner suddenly announced his retirement at the end of the month and McCarthy swiftly positioned himself as the presumptive next in line. 

Shortly after announcing his candidacy, McCarthy was seen to stumble in a Fox News interview where he appeared to link Hillary Clinton's dropping poll numbers to the congressional Benghazi committee. His comments fueled Democratic charges that the committee is merely political, which GOP leaders deny. 

McCarthy himself has walked back the comments, and the leader of that committee, South Carolina Rep. Trey Gowdy, told MSNBC on Wednesday that "Kevin screwed up." He also noted McCarthy had "apologized" for the remark. 

Amid the backlash over McCarthy's Benghazi remarks, Chaffetz entered the leadership race over the weekend. Boehner also decided to postpone other leadership elections until after the Oct. 29 full House vote for speaker. 

Whether McCarthy can rally the GOP caucus behind him is an open question. 

Republicans have nearly 250 members in the House and on paper have the numbers to win against the Democrats' nominee, likely Nancy Pelosi. But if the winning Republican nominee on Thursday comes out with a tally short of 218, he'll have to spend the next several weeks trying to rally support to get to that number. 

In a curious development, Rep. Walter Jones, R-N.C., also sent a letter to House Republican Conference Chairwoman Cathy McMorris Rodgers, R-Wash., urging a full vetting of all leadership candidates to avoid a repeat of 1998, when the conference selected then-Rep. Bob Livingston in November to succeed outgoing House Speaker Newt Gingrich. It then emerged Livingston had been conducting an affair. Jones asked that any candidate who has committed "misdeeds" withdraw. 

Asked by FoxNews.com to elaborate, Jones said he doesn't "know anything" specific about any of the candidates, but, "We need to be able to say without reservation that 'I have nothing in my background that six months from now could be exposed to the detriment of the House of Representatives.'" He said he wants to make sure the candidates have "no skeletons." 

Fox News' Chad Pergram and FoxNews.com's Cody Derespina and The Associated Press contributed to this report.