WASHINGTON—The White House is girding for more than a week of battle with Congress over President Barack Obama‘s plan to launch limited military strikes against the Syrian regime for its alleged use of chemical weapons last month.

To back the administration’s position, Secretary of State John Kerry said Sunday that the U.S. had obtained new blood and hair samples from inside Syria that confirmed President Bashar al-Assad’s regime used sarin, a powerful nerve agent, against civilians in an Aug. 21 attack on an eastern Damascus suburb.

Mr. Kerry said he believed this new evidence will help the White House build more support on Capitol Hill and among allies in Europe and the Middle East to take military action aimed at degrading Mr. Assad’s ability to conduct chemical warfare.

The leaders of the House and Senate said they would hold votes on the need for military action in Syria during the week of Sept. 9.

“If the United States is unwilling to lead a coalition of people who are prepared to stand up for the international norm with respect to chemical weapons that’s been in place since 1925, if we are unwilling to do that, we will be granting a blanket license to Assad to continue to gas,” Mr. Kerry said on ABC’s “This Week.” “We will send a terrible message to the North Koreans, Iranians and others who might be trying to read how serious is America.”

Mr. Obama said on Saturday that he has decided he should order a limited military strike against Syria, but in an about-face he said he would ask Congress to authorize the mission.

Leading Republican and Democratic lawmakers said over the weekend that Mr. Obama is going to face a major political fight to gain Congress’s approval, which requires the majority of both the House and Senate to vote in favor of military action.

Mr. Assad’s regime, meanwhile, mocked Mr. Obama’s decision to seek congressional approval for military action, claiming the U.S. had lost its nerve and was fading as a global superpower.

Diplomats and activists in Europe said they would use the congressional debate in Washington to try to build up even greater opposition to military action in Syria.

France has so far been the only nation in Europe to say it will support Mr. Obama in attacking Syria, with the U.K., Germany and Italy saying they wouldn’t participate in any military intervention. But politicians in Paris said they would use the next 10 days to try to block President François Hollande’s decision to support Washington.

“Like in the U.S. and the U.K., the president must organize a formal vote in Parliament,” Jean-Louis Borloo, head of France’s UDI centrist party, tweeted in his account Sunday.

Opposition on Capitol Hill to Mr. Obama’s plans for Syria emerged across party lines and among many conservatives and liberals.

Leading hawks on Syria, including John McCain (R., Ariz.) and Lindsey Graham (R., S.C.), said they would oppose Mr. Obama’s policy if it didn’t go far enough in weakening Mr. Assad’s regime and preparing for a change of government in Damascus.

Opponents of military action said the bombing of Syria risked quickly escalating and bringing in Mr. Assad’s chief international backers, Iran and the Lebanese militant group Hezbollah.

They also worried that a U.S. attack on Syria would serve only to ensure a stalemate in the Arab country while continuing to flooding its neighbors with refugees.

“I’m not sending my son, your son, to fight for a stalemate,” said Sen. Rand Paul, (R., Ky.) on NBC’s “Meet the Press.”

Neither House nor Senate leaders said they were formally calling their members back to Washington before the scheduled Sept. 9 end of their August recess, but House members got a classified briefing on the Syria crisis Sunday afternoon with officials from the White House, the State Department, the Defense Department and the Office of the Director of National Intelligence.

The Senate Foreign Relations Committee will be reconvening early to hold a hearing Tuesday on the resolution to authorize the use of force, but there could be more than one. Foreign Relations Chairman Bob Menendez (D., N.J.) said “senior administration witnesses will testify” this week.

Both the politics and the logistics of moving the use-of-force resolution forward in Congress are difficult. It remains unclear exactly what measure the House and Senate will be voting on because lawmakers are likely to seek changes in the draft submitted by the White House on Saturday night.

A senior aide to House GOP leaders said they regarded the White House draft sent to Congress Saturday as a “starting point” for discussions among lawmakers. “Our committees and staff will look over it and decide the best way to proceed,” said the GOP aide, who added that the measure will be reviewed by three committees—Foreign Affairs, Armed Services and Intelligence.

Mr. Kerry and other senior U.S. administration officials worked on Sunday to counter the charges being made by some American lawmakers and allies that Mr. Obama backed down on his pledge to use force, and buckled under political and diplomatic pressure.

A key question is who will be leading the charge on Capitol Hill to help build support for the measure in advance of the vote. House Speaker John Boehner (R., Ohio) hasn’t stated his position but has raised questions about the administration’s broader goal and strategy.

However, Mr. Obama has an important Republican ally in House Intelligence Chairman Mike Rogers, who has strongly supported a targeted strike on Syria.

“We better send a very clear message, in a unified way, that we’re not going to tolerate proliferation of weapons of mass destruction, let alone their use,” Mr. Rogers said Sunday on CNN.

Israeli and Arab officials, who have been the most aggressive in seeking to punish Mr. Assad, said Sunday they worried Washington’s delay could end up emboldening Damascus, as well as Iran and Hezbollah. “Obama wants to stay neutral on everything—Egypt, Syria,” said a senior Arab official working on the Syrian conflict. “Sometimes middle ground doesn’t work.”

Mr. Kerry gave an impassioned speech on Friday outlining the need to act against Mr. Assad’s forces. But he and other U.S. officials argued Sunday that taking the time to build greater consensus in Washington ultimately would strengthen the case against Damascus. They said the growing amount of intelligence documents the Syrian regime’s use of sarin would also aid Mr. Obama’s policy.

Mr. Kerry was among the handful of senior administration officials who held extensive discussion at the White House before the president announced his decision to seek congressional approval on Saturday afternoon. U.S. officials said they sought to learn from the situation in the U.K., where Prime Minister David Cameron sought to quickly obtain legislative support for Syria strikes, but failed.

“I believe that as we go forward in the next days, the Congress will recognize that we cannot allow Assad to be able to gas people with impunity,” Mr. Kerry told ABC.

The secretary of state was cautious in detailing what the administration would do if Congress voted against strikes on Syria. But he said the White House believed Mr. Obama was within its rights to launch the attacks without congressional approval.

The administration has taken great extensive steps over the past two weeks to provide classified information to Congress and the press detailing the Assad regime’s alleged use of sarin gas on Aug. 21, which the U.S. estimates killed some 1,500 people, including more than 400 children.

Last week, the White House released a four-page intelligence report that claimed Syrian security forces extensively planned for the attack on the Damascus suburb of East Ghouta, citing human intelligence, communications intercepts and social-media reports. U.S. officials said Syrian forces appeared to be trying to clear the neighborhood of rebels so that they could launch strikes on the strategic city of Aleppo.

Mr. Kerry on Sunday said the U.S.’s acquisition of blood and hair samples only further solidified the U.S.’s belief that the Assad regime used sarin gas.