President Obama’s announcement Saturday that he will seek congressional approval for a possible attack on Syria sets the stage for the most tumultuous foreign policy debate on Capitol Hill in more than a decade.

And already, an unlikely alliance between tea party conservatives and veteran liberal doves, as well as the memory of the Iraq war debate, has cast doubt on whether the president can mobilize enough support in the country and in Congress to persuade lawmakers to approve even a limited attack in Syria.

After Obama’s decision Saturday to seek congressional approval for the military action, House and Senate leaders began laying the groundwork for votes on the use of military force in response for the suspected use of chemical weapons by the Syrian government.

The Senate will hold committee hearings this coming week and a full debate and vote the following week, Majority Leader Harry M. Reid (D-Nev.) announced Saturday evening, while the House plans to stick to its original schedule of returning Sept. 9 and deliberating the use-of-force request that week. Much of the mechanics of the debate and the exact nature of the request remained unclear; Obama did not inform congressional leaders of his decision until Saturday morning.

Lawmakers and aides suggested that the president could face an uphill fight in winning approval in both chambers, particularly in the House. There, the Democratic caucus is dominated by liberal veterans who are still bitter about the passage of the 2002 Iraq war resolution based on faulty intelligence of Saddam Hussein’s ultimately nonexistent stockpiles of chemical weapons, and the House GOP caucus has an increasingly potent wing of isolationist lawmakers wary of overseas entanglements.

Timeline: Unrest in Syria

Two years after the first anti-government protests, conflict in Syria rages on. See the major events in the country's tumultuous uprising.

Even Republicans who have worked closely with Obama on foreign policy and fiscal issues in recent months warned that his administration had hard work ahead.

“Now that the president has decided to use force and seek authorization, it is imperative that he immediately begins using every ounce of his energy to make his case to the American people,” said Sen. Bob Corker (Tenn.), who is the top Republican on the Foreign Relations Committee and is generally supportive of a strike.

Geared up for fiscal debates

The debate, which is certain to be emotional on both sides, comes as Congress is supposed to begin a heated faceoff on fiscal matters. Leaders hope to pass an interim spending bill by Sept. 30 to keep the federal government operating, while facing another deadline a few weeks after that to lift the Treasury’s borrowing authority, or else risk a default on the nation’s $16.7 trillion debt. The White House and leading Senate Republicans, including Corker, declared an impasse late last week on those fiscal talks.

The request for congressional approval followed calls from both sides of the partisan aisle for a vote similar to the one held last week in Britain, where Parliament voted down Prime Minister David Cameron’s request to join U.S. forces in a strike in Syria.

By the end of the week more than 140 House members, including more than 20 Democrats, had signed a letter drafted by Rep. Scott Rigell (R-Va.) demanding that Obama get congressional approval before launching any action.

A different letter demanding debate before a strike, penned by Rep. Barbara Lee (D-Calif.), the leading anti-war liberal, gained more than 60 Democratic signatures.

House Minority Leader Nancy Pelosi (D-Calif.) has emerged as the strongest supporter of Obama’s plan. “Military action in response to Assad’s reckless use of deadly gas that is limited in scope and duration, without boots on the ground, is in our national security interest and in furtherance of regional stability and global security,” Pelosi said in a statement, referring to Syrian President Bashar al-Assad.

Pelosi has demonstrated an ability to deliver votes when the Obama administration has needed them, but other Democrats said Syria will be a tougher sell because many still feel stung by the Iraq war votes 11 years ago.

“The shadow of Iraq is the dominant influence for most members,” said Rep. Gerald E. Connolly (D-Va.). Connolly said he is also concerned with historic parallels to massacres in the 1990s that did not get a forceful response. “Syria is not Iraq, and Obama is not Bush,” he said. “No one is contemplating an invasion of Syria. The appropriate analogies are Bosnia-Kosovo-Serbia and Rwanda. The tug of historical analogies is a powerful force in foreign policy.”

House Speaker John A. Boehner (R-Ohio) and his top lieutenants issued a noncommittal statement, praising the president for allowing Congress to weigh in but not assuring how they would vote.

House members have been invited to a classified briefing in the Capitol on Sunday, when they can review more-detailed intelligence about the Syrian attacks than they have received in a series of conference calls with administration officials in recent weeks. It is unclear how many lawmakers will be able to make it back to Washington on short notice for the intelligence briefing.

Clearer path in Senate?

The Senate, where muscular foreign policy views still tend to hold sway, is believed to be an easier lift for Obama. Senior senators such as Robert Menendez (D-N.J.), Bill Nelson (D-Fla.) and Johnny Isakson (R-Ga.) were among the few who issued forceful statements Saturday supporting the president’s plan for limited military strikes.

However, a potential blow came from the Senate’s most prominent hawks, John McCain (R-Ariz.) and Lindsey O. Graham (R-S.C.), who said that the president had not gone far enough. McCain and Graham issued a joint statement hinting that they would oppose the resolution because the limited strikes Obama was seeking to have authorized would not “change the momentum on the battlefield” and would leave Assad in place — instead of the regime change Obama had previously stated was his goal.

“Anything short of this would be an inadequate response to the crimes against humanity that Assad and his forces are committing,” McCain and Graham said.

Without their support, it is unclear how many Republican votes there will be for the use-of-force resolution, particularly with the small but vocal wing of libertarian Republicans appearing poised to firmly oppose the vote.

“I am encouraged President Obama now says he will fulfill his constitutional obligation to seek authorization for any potential military action in Syria,” Sen. Rand Paul (R-Ky.), who is considering seeking the GOP presidential nomination in 2016, said in a statement. “This is the most important decision any President or any Senator must make, and it deserves vigorous debate.”

Senate Minority Leader Mitch McConnell (R-Ky.), who is closely relying on Paul’s support for his 2014 reelection bid, has remained silent for the past week on his position on Syria.

Speaking to reporters after his speech at the Americans for Prosperity Foundation summit in Orlando, Sen. Ted Cruz (R-Tex.) said he still needs to be convinced that it is in the country’s interests to take action. “It is incumbent on the president to make the case that military action is in furtherance of the vital national security interests of the United States,” said Cruz, also a possible 2016 contender. “I am troubled by the justifications that the Obama administration has put forth so far. Much of their discussion has concerned what they described as international norms.”

Matea Gold contributed to this report.