WASHINGTON – Leading lawmakers dealt bipartisan rejection Sunday to President Obama’s request to strike Syrian military targets, saying the best hope for congressional approval would be to narrow the scope of the resolution.

From the Democratic dean of the Senate to tea party Republicans in their second terms, lawmakers said the White House’s initial request to use force against Syria will be rewritten in the coming days to try to shore up support in a skeptical Congress.

But some veteran lawmakers expressed doubt that even the new use-of-force resolution would win approval, particularly in the House.

“I think it’s going to be a very tough sell,” said Rep. Tom Cole, Okla., who is often a key crossover Republican in compromises with the White House. For now, Cole said he is “leaning no” on approving any use of force against Syria.

His remarks came after a more than 2 1/2-hour classified briefing that drew nearly 100 lawmakers to the Capitol, flying in from across the country on 24 hours’ notice for a rare Labor Day weekend meeting.

The closed-door briefing, run by five senior national security officials, began the administration’s all-out effort to win support for what Obama has said would be a limited strike against military targets to punish Syrian President Bashar Assad’s regime for carrying out a chemical attack.

White House officials have less than two weeks to secure backing in the House and the Senate, which will not formally return from their regular end-of-summer break until Sept. 9. They are expected to then immediately begin debate on military authorization, with votes by mid-September.

Secretary of State John Kerry, who has been talking to his former colleagues in the Senate, predicted victory during appearances on five Sunday talk shows.

Lawmakers from both parties said Sunday that the administration has presented convincing evidence that Assad’s government carried out the attack, citing Sunday’s briefing and other classified presentations that they received in the past week. The key stumbling block, they said, was the concern that a limited strike would not be an effective deterrent and would only draw the U.S. military deeper into Syria’s civil war.

“I don’t think there’s a lot of doubt that the regime undertook this attack. There’s a great deal of skepticism that a limited strike is likely to be effective,” said Rep. Jim Himes, D-Conn., a member of the House Intelligence Committee.

The uncertain outcome is rooted in a Congress that has proved deeply factionalized and dysfunctional. With Democrats running the Senate and Republicans the House, the two sides have fought to a near legislative standstill on almost every major issue. A proposal to stiffen background checks for gun buyers died in the Senate this spring, despite having the support of 90 percent of the public. A comprehensive overhaul of immigration laws, also backed by a majority of voters, has stalled.

Add to that mix a heated debate on something as consequential as war and its constitutional underpinnings, and the atmosphere on Capitol Hill could grow even more toxic.

The most difficult hurdle comes in the House, which has been incapable this year of approving what in the past were considered perfunctory measures. The farm bill, usually a bipartisan celebration of agriculture policy, failed in late June.

Aware of the growing bloc of Republican isolationists, senior Republican aides warned Sunday that a large number of Democrats will have to support the use-of-force resolution for it to have any chance.

Obama’s allies said the first order of business will be to work with the administration to redraft the resolution, which was sent to Capitol Hill on Saturday night and barely filled one page. It had no prescriptions for what type of military action could be carried out or its duration.

Sen. Patrick Leahy, D-Vt., the dean of the Senate and chairman of the Judiciary Committee, told reporters that the resolution is “too open-ended” as written. “I know it will be amended in the Senate,” he said.

Rep. Chris Van Hollen, D-Md., a former Senate staffer who inspected chemical weapons attacks by Saddam Hussein’s government against its own citizens in Iraq in the 1980s, said he will push to add language that would limit the length of the mission and prohibit putting U.S. troops on the ground in Syria.

Such provisions could gain support from lawmakers who want to rein in the Obama administration, without hampering the goals of the mission — which the president has said should be limited to missile strikes against military targets.