Ryan said he would not run for the chamber's top job unless he got endorsements from all three of the major House GOP factions: the hard-line Freedom Caucus, conservative Republican Study Committee and centrist Tuesday Group.

The Republican Study Committee and Tuesday Group endorsed him Thursday, while the Freedom Caucus said only that a supermajority of its members would back Ryan.

Still, Ryan indicated that was enough for him to move forward after more than a week of consideration.

“I never thought I’d be speaker. But I pledged to you that if I could be a unifying figure, then I would serve — I would go all in. After talking with so many of you, and hearing your words of encouragement, I believe we are ready to move forward as a one, united team. And I am ready and eager to be our speaker,” Ryan wrote in a letter to House Republicans on Thursday evening.

House Republicans will vote behind closed doors Oct. 28 to nominate a Speaker. A full House vote to formally elect a new Speaker is slated for the following day.

Ryan had previously rejected pleas from colleagues to run for the position after Rep. John Boehner (R-Ohio) announced he would step down as Speaker and resign from Congress by the end of October. But after Majority Leader Kevin McCarthy (R-Calif.) abruptly dropped out of the race earlier this month, pressure grew on Ryan to run.

Ryan has repeatedly said the Speakership is not a job aspired to. The 45-year-old Ways and Means Committee chairman had long been expected to run for president someday, a goal that may be complicated by becoming the highest-ranking official in Congress.

Instead, the 2012 GOP vice presidential nominee will run for a job that would make him second in line for the presidency.

Another factor causing Ryan’s reluctance to take the job is his family in Janesville, Wis. Ryan has said he wants to spend less time on the road fundraising than most Speakers do and instead spend more time communicating the party’s message.

Ryan at this point appears to have the 218 votes needed to win the Speaker election on the floor.

About 70 percent of the roughly 40-member Freedom Caucus is behind Ryan. That fell short of the 80 percent threshold necessary for the Freedom Caucus to make an official endorsement.

Ryan has been telling his GOP colleagues he’s open to changing some internal rules and procedures as requested by conservatives. He supports changing the membership of the Steering Committee, which selects committee chairmen and assignments, according to sources who attended Ryan’s meeting with the Freedom Caucus on Wednesday.

Ryan also wants to change the rules for the "motion to vacate the chair," which allows any member to force a referendum vote to oust a sitting Speaker. Rep. Mark Meadows’s (R-N.C.) introduction of a motion in July was as a factor in Boehner’s decision to resign.

In addition, Ryan pledged to the Freedom Caucus that no immigration reform bill would come to the floor unless it had support from a majority of the majority party.

Boehner plans to step down from Congress Oct. 30, a day after the Speaker election.

Updated at 7:12 p.m.