WASHINGTON -- U.S. House Republican moderates lined up Thursday behind Rep. Paul Ryan's bid to become next House speaker, a day after backing from influential hardline conservatives made it all but certain that the party's 2012 vice-presidential nominee will take the chamber's top job and become second in line to the presidency.

"We all support Paul Ryan," Rep. Carlos Curbelo, a member of the Tuesday Group of House Republican moderates, said after meeting with the Wisconsin Republican. His backing from the group was considered a foregone conclusion.

Ryan, who was lobbied by establishment Republicans to seek the post, cleared his biggest hurdle late Wednesday when a majority of hardline conservatives on the pugnacious House Freedom Caucus said they favoured his ascension to the speakership. That verdict was a relief for many Republicans hoping Ryan's rise would end a month of political pandemonium that has made Congress and the Republicans seem anarchic.

Around two-thirds of the Freedom Caucus' three dozen members voted to back Ryan to lead the House, caucus members said. With their support the chief remaining question over whether he'd seek the job, the group's vote essentially meant the speaker's post is his for the taking when the House formally chooses a new leader next week.

Republicans believe that by assuming command of the House, Ryan could stem the political fratricide between the party's pragmatists and hard-right conservatives that helped force last month's startling resignation by Speaker John Boehner. That conflict has also colored the Republican 2016 presidential contest, where leading candidates Donald Trump and Ben Carson have positioned themselves as outsiders and reveled in attacking establishment Republicans as relics of an outmoded era.

The Freedom Caucus' vote fell short of the 80 per cent margin the group requires for an official endorsement, which Ryan had earlier demanded as a condition of seeking the post. But in a written statement afterward, Ryan erased any doubt that he considered their vote a green light to run.

"I believe this is a positive step toward a unified Republican team," Ryan said.

In their own statement, the Freedom Caucus praised Ryan as "a policy entrepreneur who has developed conservative reforms dealing with a wide variety of subjects." They said that they had not reached agreement with Ryan over conditions he set for serving -- including curbing lawmakers' ability to call snap votes to remove a speaker -- but said those differences could be resolved "in due time."

House Minority Leader Nancy Pelosi, a Democrat, told reporters Thursday that Ryan is someone "who knows the territory, knows the issues, so that's helpful." But she noted that as chairman of the House Budget Committee, Ryan produced fiscal plans that reshaped a health care progam with voucher-like payments, warning, "So seniors should be watching the Ryan priorities carefully."

Ryan's endorsement by the mainstream conservative Republican Study Committee, another key group of House Republicans, is also considered a formality.

The Freedom Caucus tilted toward Ryan even as many Republicans suggested the group would face dire consequences if it derailed him. The 45-year-old is widely viewed within the party as an articulate, telegenic leader, an undisputed conservative and the only lawmaker who could unite a party divided since the 2010 tea party wave brought rebels to Congress who have shown little inclination to compromise.

AP Congressional Correspondent Erica Werner contributed to this report