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 remain open for debate and are expected to take several days.

The day after Obama's national address, GOP leaders were mulling exactly how to handle the president's request to explicitly authorize the training and arming of foreigners to combat the Islamic militants.

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. He said that many Republicans are skeptical of the policy laid out by Obama.

"If our goal is to eliminate ISIL, there is a lot of doubt of whether the plan that was outlined last night will accomplish that," he said, but added later: "It’s important to give the president what he has asked for."

Boehner also cautioned that more may be needed: “A F-16 is not a strategy, airstrikes alone will not accomplish what we want to accomplish," he said.

Despite his public comments, Boehner told a breakfast gathering of House Republicans at the U.S. Capitol that he supports Obama's request to arm rebels in the war-torn region, according to several attendees. His early endorsement is a boost to the White House, signaling congressional approval is likely.

Meanwhile in the Senate, there were growing calls for Congress to debate the issue, and Majority Leader Harry M. Reid (D-Nev.) urged Congress 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.

The fresh debate over how to respond to Obama's military campaign has upended the carefully-organized September calendar in the House and Senate. Top leaders had hoped to convene for two weeks to quickly approve a short-term spending measure, reauthorize a handful of federal programs and perhaps confirm a few more Obama administration nominees before going home through the elections.

But a handful of House and Senate hearings on the Islamic State threat are now scheduled for early next week and any vote to authorize military action isn't expected until after those proceedings. That leaves open the possibility that the House and Senate will need to abandon plans to adjourn as early as next Friday to allow incumbents to head home for the remainder of the election season.

With a vote still days away, members of the House and Senate were attending closed-door classified briefings Thursday with national security officials. Among those participating in the briefings is Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff and Tony Blinken, one of Obama's deputy national security advisers.

Heading into a closed-door briefing for all House lawmakers Thursday morning, partisanship was evident, if muted because of the anniversary of the Sept. 11, 2001 terrorist attacks.

Republicans said they worried that by only asking for the legal authority to train and equip Syrian rebels, Obama was unnecessarily limiting his ability to wage war and said they would push for specifics on how the president will potentially expand the mission if the situation worsens.

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

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

Several lawmakers said they expect more debate and positioning in the coming days even as both sides push to pass a bipartisan package. Separating legislation to fund the government and the authorization was an idea that drew new support throughout the late morning and early afternoon as House members grappled with the political implications of combining it.

Top House GOP aides said doing so could cost Boehner some Democratic votes, since many are inclined to firmly support the president’s desire to pin the two proposals together. But voting on a separate measure would keep many conservatives on board, who made clear to the leadership during meetings Thursday that they do not want authorization to be part of the larger spending bill due to skepticism among conservative leaders and grassroots activists.

Leaving the GOP breakfast meeting, several rank and filemembers mentioned possibilities for how legislation could soon reach the House floor, with the option of a standalone vote gaining some traction. Others, including some tea party members, said they would follow Obama’s urging on Wednesday to insert authorization for his strategy into a spending bill that would also keep open the U.S. government, which has its funding set to expire at the end of September.

“Either way is fine by me, as long as we do something,” said Rep. Andy Harris (R-Md.), a hardline conservative. “I have no preference other than confronting ISIS.”

Rep. John Shimkus (R-Ill.) said he was mixed about Obama’s Wednesday remarks but would fully support the president’s request for congressional approval of funding to train fighters from Iraq, Syria, and other U.S. allies.

A group of younger libertarian doves sounded unhappy with their colleagues’ embrace of the president’s push for military action, and wondered aloud 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.

Sen. Joe Manchin III (D-W. Va.) said he would prefer to authorize Obama's plans to train and equip Syrian rebel groups and Iraqi military forces in a standalone measure instead of as part of the short-term spending bill, commonly referred to as a continuing resolution or "CR" on Capitol Hill.

"If it’s wrapped up into the CR, you’re voting against it, you’re voting to shut down the government and that’s not a fair position to put us in," he said in an interview with The Post Wednesday night. "I think we should vote straight up or down with the ability to modify it. We should have some input in it. "

Sen. Angus King (I-Maine), who caucuses with Democrats and could emerge as a key power broker in a potentially GOP-controlled Senate next year, said that he and a "coalition of bipartisan senators" are talking about drafting a resolution that would give President Obama limited authority to conduct military operations against the Islamic State terror group.

King said that "about a dozen" senators of both parties had begun discussions Wednesday afternoon about possibly drafting a resolution that would set some limits. But he declined to specify what they would be, suggesting that the talks are still fluid. Regardless of the political challenge, King said Congress should take up the issue before the November elections.

"I think it strengthens the country and the effort and it sends a strong signal to the proposed allies that we’re in this," he said in an interview Wednesday night, adding later: "We have a constitutional responsibility to be engaged in this. I’m frustrated by Congress’s propensity to criticize and not make decisions. This is an opportunity where we should engage in this. I think it strengthens the country if we do so."

Wesley Lowery and Paul Kane contributed to this report.