President Obama won the support on Tuesday of Republican and Democratic leaders in the House for an attack on Syria, giving him a foundation to win broader approval for military action from a Congress that still harbors deep reservations.
Speaker John A. Boehner, who with other Congressional leaders met Mr. Obama in the Oval Office, said afterward that he would “support the president’s call to action,” an endorsement quickly echoed by the House majority leader, Representative Eric Cantor of Virginia.
On Tuesday evening, Democrats and Republicans on the Senate Foreign Relations Committee agreed on the wording of a resolution that would give Mr. Obama the authority to carry out a strike against Syria, for a period of 60 days, with one 30-day extension. A committee vote on the measure could come as early as Wednesday.
Uncertainties abound, particularly in the House, where the imprimatur of the Republican leadership does not guarantee approval by rebellious rank and file, and where vocal factions in both parties are opposed to anything that could entangle the nation in another messy conflict in the Middle East.
Still, the expressions of support from top Republicans who rarely agree with Mr. Obama on anything suggest the White House may be on firmer footing than seemed the case on Saturday, when the president abruptly halted his plans for action in the face of growing protests from Congress.
Mr. Obama is now headed to Sweden and Russia, where he will try to shore up an international coalition to punish Syria for a chemical weapons attack and will probably encounter some of the same debates that are cleaving the Capitol.
Before his departure, the White House intensified what has become the most extraordinary lobbying campaign of Mr. Obama’s presidency as it deployed members of his war council and enlisted political alumni of his 2008 campaign to press the argument with the public.
“This is not the time for armchair isolationism,” said Secretary of State John Kerry, who answered sharp questions and defended the administration’s strategy for Syria in nearly four hours of sometimes sharp exchanges before the Senate Foreign Relations Committee.
Mr. Kerry stirred some confusion about the potential scope of American military involvement when he tried to carve out an exception to a proposed Congressional prohibition on the use of ground troops in Syria — something Mr. Obama and other officials have long ruled out as a general principle.
If Syria were to fall into complete chaos and if the chemical weapons of President Bashar al-Assad’s government there were at risk of falling into the hands of a militant group like Al Nusra, Mr. Kerry said, “I don’t want to take off the table an option that might or might not be available to a president of the United States to secure our country.”
Later, under questioning by Senator Bob Corker of Tennessee, the ranking Republican, Mr. Kerry walked back his comment, insisting that he had only been speaking about a hypothetical case. “Let’s shut that door now as tight as we can,” Mr. Kerry said, without quite doing so. “There will not be American boots on the ground with respect to the civil war.”
The Senate resolution — released Tuesday night by Mr. Corker and the committee’s chairman, Robert Menendez, Democrat of New Jersey — would limit the president’s options and prohibit the use of ground forces. Any strike, it says, should be “tailored” to only deter Syria from using chemical weapons again and to cripple its capacity to do so.
The resolution would prohibit “boots on the ground” and require “the Obama administration to submit their broader plan for Syria,” Mr. Corker said in a statement.
Mr. Menendez added, “We have an obligation to act.”
In one of the most heated moments of the hearing earlier, Senator Rand Paul, the Kentucky Republican, said that Mr. Obama might go through with an attack if Congress failed to authorize it. Mr. Kerry said that he did not know what Mr. Obama would decide but that the president had the authority to do so under the Constitution.
It was a vivid tableau: Mr. Kerry — the former senator and chairman of the Senate Foreign Relations Committee, who voted to authorize the Iraq war in 2003, then turned against it — imploring his ex-colleagues to authorize an act of war.
Although he appeared alongside Defense Secretary Chuck Hagel — another former senator — and the chairman of the Joint Chiefs of Staff, Gen. Martin E. Dempsey, Mr. Kerry dominated the hearing. He seemed keenly aware of the echoes of Iraq.
“We were here for that vote,” Mr. Kerry said. “We voted. So we are especially sensitive — Chuck and I — to never again asking any member of Congress to take a vote on faulty intelligence. And that is why our intelligence community has scrubbed and rescrubbed the evidence.”
Mr. Kerry said the intelligence proved that the “Assad regime prepared for this attack, issued instructions to prepare for this attack, warned its own forces to use gas masks,” and the intelligence included “physical evidence of where the rockets came from and when.”
Mr. Hagel, who, like Mr. Kerry, is a veteran of the Vietnam War, used another argument used by previous administrations: a warning that authoritarian governments with arsenals of unconventional weapons could transfer them to terrorist groups.
Casting the issue as one of self-defense, the defense secretary also underscored the threat to American military personnel across the region. He said other dictators around the world and militant groups like Hezbollah might be emboldened if the United States did not punish the Assad government. “The use of chemical weapons in Syria is not only an assault on humanity,” Mr. Hagel said. “It is a serious threat to America’s national security interests and those of our closest allies.”
Before the hearing began, and again after Mr. Kerry spoke, protesters from the antiwar group Code Pink jumped up and shouted against military action. “Kerry, no more war in Syria!” one demonstrator exclaimed, adding that America needed health care and education more than military action.
Although the declared goal of a strike on Syria would be to degrade its ability to launch a chemical weapons attack and deter any future use, General Dempsey was asked whether such an attack would also diminish to a broader extent the Assad military’s abilities.
“Yes,” he replied.
General Dempsey was a subdued presence in the hearing. Although he, Mr. Kerry and Mr. Hagel sought to present a unified front, they have had differences over how to respond to the conflict in Syria in recent months. Mr. Kerry has pushed to provide military support to the rebels and consider deeper military involvement, and General Dempsey has repeatedly highlighted the risks of intervention.
Similar differences were on display among lawmakers who spoke during the Senate hearing or after the meeting with Mr. Obama, Mr. Kerry and Vice President Joseph R. Biden Jr.
Representative Nancy Pelosi of California, the House minority leader, said she supported the president and sent a letter to fellow Democrats urging that they fall into line. But she conceded, “In my district, I don’t think people are convinced that military action is necessary.”
Ms. Pelosi’s comments reflected her dilemma as a leader of the president’s party, which still has a strong liberal antiwar wing. “The American people need to hear more about the intelligence,” she said.
A spokesman for Mr. Boehner said that despite his support for Mr. Obama, the Republican leadership would not lean on other Republicans to vote for military action and would leave that lobbying to the White House. Mr. Boehner’s stance will ease the pressure on him from members of his party, who believe the United States has no business in Syria. It will increase the pressure on Ms. Pelosi.
The calendar is Mr. Obama’s enemy: Many members from both parties are still back in their districts hearing from constituents, and the feedback, based on numerous interviews, is overwhelmingly negative.
On Tuesday, however, a powerful pro-Israel lobbying group, the American Israel Public Affairs Committee, threw its support behind military action in Syria, citing the need to send a strong message to Iran and the militant group Hezbollah, both of which support Mr. Assad.
“Iran is watching us very carefully,” said Representative Eliot L. Engel, Democrat of New York and a staunch defender of Israel.