Here are key moments from the speaker of the House's news conference following President Obama's speech on America's efforts to combat Islamic State militants. (AP)

House Republican leaders moved quickly Thursday to broadly support President Obama’s plan for an open-ended campaign to combat the Islamic State — but the mechanics of how they will do so won’t be determined for several days.

Congress is “at the beginning stages of building the kind of support that is needed across the nation to carry out this plan,” House Speaker John A. Boehner (R-Ohio) told reporters Thursday as he cautioned that many Republicans are skeptical of Obama’s strategy.

“If our goal is to eliminate ISIL” — an acronym commonly used for the radical group — “there is a lot of doubt whether the plan that was outlined last night will accomplish that,” Boehner said. But he added later, “It’s important to give the president what he has asked for.”

Boehner spoke after telling a breakfast meeting of House Republicans that he supports Obama’s request to arm moderate rebels in war-torn Syria, according to several attendees. His endorsement is a boost to the White House, signaling congressional approval is likely.

There was growing support among lawmakers Thursday for Congress to debate the issue, but Senate Majority Leader Harry M. Reid (D-Nev.) urged colleagues not to be “rash” in approving legislation that they might regret later. “We have to be deliberate,” he told reporters at a news conference as he explained why Congress should not take up a full use-of-military-force resolution this fall.

President Obama's language on war has evolved throughout his time in office. From his Nobel Peace Prize speech in 2009 to his remarks on the Islamic State, here are his remarks over the past six years. (Nicki DeMarco/The Washington Post)

House Minority Whip Steny H. Hoyer (D-Md.) became the ­highest-ranking congressional leader to suggest that a full war debate will take place. He proposed a “two-step process” to handle the issue that would begin with Congress approving a modest proposal authorizing Pentagon officials to train and arm pro-Western rebels inside Syria. Then, Hoyer said, the lame-duck session would provide time for expansive debate and resolutions providing clear war language and setting parameters for U.S. forces carrying out the attacks.

Many lawmakers “believe that we ought to have a vote before the end of the year on the authorization of the use of military force so that the Congress can speak and represent the views of the American people,” Hoyer said during an interview for C-SPAN’s “Newsmakers” program, airing Sunday.

The idea of holding an up-or-down vote to authorize Obama’s military strategy rather than tucking the authorization into a short-term spending bill — as the White House has suggested — drew wide support as lawmakers grappled with the political implications of combining the two measures.

“I think we should vote straight up or down with the ability to modify it. We should have some input in it,” Sen. Joe Manchin III (D-W.Va.) said in an interview. Later, he told reporters that he has “deep concerns about us investing taxpayers’ dollars, at $500 million, into a group of rebels when we are not sure how it will be used to our benefit.”

Reid and Boehner declined to say Thursday how they might proceed. House GOP aides said holding a separate vote could cost some Democratic support, since many are inclined to firmly support Obama’s desire to pin the two proposals together. But voting on a separate measure would earn the backing of many conservatives, who told House GOP leadership during meetings Thursday that they do not want authorization to be part of the spending bill because of skepticism among conservative leaders and grass-roots activists.

The congressional response to Obama’s new military campaign has upended the carefully organized September calendar. Leaders originally planned for a quick two weeks to approve the short-term spending measure and re­authorize a handful of federal programs. But several House and Senate hearings on the Islamic State are now scheduled for early next week, and any vote to authorize military action isn’t expected until after those proceedings.

The House is set to reconvene Monday — earlier than scheduled — to give House members more time to consider Obama’s request and the short-term spending bill to keep the federal government funded when the new fiscal year begins Oct. 1. House Majority Leader Kevin McCarthy (R-Calif.) said Thursday that he hopes to be able to adjourn next Friday.

With a vote still days away, lawmakers attended closed-door briefings Thursday led by top national security officials, including Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, and Tony Blinken, one of Obama’s deputy national security advisers.

Some congressional Republicans said they worried that by asking only for the legal authority to train and equip Syrian rebels, Obama was unnecessarily limiting his ability to wage war.

“This president has stood by with a golf club in his hands,” said Rep. Trent Franks (R-Ariz.). He called on Obama to recognize George W. Bush for his past leadership on foreign policy and combating terrorism around the world.

A group of younger libertarian doves sounded unhappy with their colleagues’ embrace of Obama’s push for military action. They wondered whether House Republicans were rushing to rally behind a campaign that could turn sour.

“I disagree with the president,” said Rep. Thomas Massie (R-Ky.), who opposes intervention. Coupling war-related legislation with a government funding bill, he added, would be a mistake.

But Sen. Marco Rubio (R-Fla.), a frequent Obama critic, said, “I think we’re in a better place with the president than we were a few weeks ago,” adding later: “He’s come a long way. He used to say that ISIL was the JV team; now he acknowledges they’re a real threat.”

House Democrats said they would ask Republicans to back the president as soon as possible. Rep. Marcy Kaptur (D-Ohio) said, “There is always a tension in both parties at times like these, but I think it’ll happen.”

Paul Kane and Wesley Lowery contributed to this report.