Nearly a week into President Obama’s campaign to convince Congress that airstrikes against Syria are necessary, he has achieved little headway against a wall of skepticism on Capitol Hill.

The president’s challenge is made more difficult by the fact that the two parties are splintered on the issue — and that lawmakers say they are hearing virtually no support for an attack from their constituents at home.

Rep. Justin Amash (R-Mich.), a libertarian who has taken on GOP hawks on National Security Agency surveillance and now Syria, tweeted Thursday: “If you’re voting yes on military action in #Syria, might as well start cleaning out your office. Unprecedented level of public opposition.”

Democrats are torn between their fear of crippling a Democratic president with a “no” vote and their anxiety that they might be repeating the mistakes of recent history in the Middle East and Afghanistan.

For Republicans, the debate over striking Syria has reopened a long-standing schism between the GOP’s internationalist and non-interventionist wings at a moment when the party is struggling to reinvent itself. The vote will be a test of some of the party’s possible 2016 presidential contenders, who until now have had the luxury of standing on the sidelines and criticizing Obama on foreign policy.

As of Sept. 4, lawmakers appear to be tentatively dividing into four camps over military action in Syria.

Given the dissent within their ranks, even the most influential of those who back the operation are showing little enthusiasm for urging their colleagues to come aboard.

In the House — where prospects for approval appear dimmer than they do in the Senate — Speaker John A. Boehner (R-Ohio) and Majority Leader Eric Cantor (R-Va.) have said they favor strikes but will not pressure other members on what they consider a “conscience vote.”

On the Democratic side, “I’m not exactly leading the charge,” House Minority Leader Nancy Pelosi told Time magazine. “But I’m supporting the president.”

On all sides, uncertainty remains over what would be achieved by attacking Syria over its alleged use of chemical weapons.

Lawmakers remain unconvinced that limited strikes proposed by Obama would shift the balance in a bloody civil war that appears tipped in favor of President Bashar al-Assad’s government. Or whether that is, in fact, what is desired.

“In order to justify action now against his regime and risk further escalating the conflict, the president must clearly identify what our national security interests are,” said Rep. Ileana Ros-Lehtinen (R-Fla.), who heads a House Foreign Affairs Committee panel on the Middle East.

“What are our objectives in limited and targeted airstrikes? What does degradation [of the Syrian government forces] look like? And what will we do if the initial action does not yield the intended result?” she asked Wednesday at a hearing with administration officials.

The Senate Foreign Relations Committee on Wednesday approved an authorization resolution on a narrow 10-7 vote.

More telling than the total were the fault lines the vote revealed.

Those opposed included five Republicans and two of the panel’s most liberal Democrats, Tom Udall (N.M.) and Christopher Murphy (Conn.).

Meanwhile, several Republican establishment figures — John McCain and Jeff Flake of Arizona, as well as Bob Corker (Tenn.) — sided with the Democratic majority. That put them on the opposite side from two potential 2016 presidential contenders, Rand Paul (Ky.) and Marco Rubio (Fla.).

So muddled was the vote that it may not have the influence it might have had with senators outside the committee.

Sen. Joe Manchin III (D-W.Va.) said he attended the committee’s hearing even though he is not a member. In addition, he has gone to hearings of the Senate Armed Services Committee, on which he sits; attended classified briefings by administration officials and sought the advice of experts.

“In good conscience, I cannot support the Senate Foreign Relations Committee’s resolution and will be working with my colleagues and the administration to develop other options,” Manchin said in a statement released by his office Thursday.

Both parties are deeply divided — a rare event in these hyperpartisan times — but for different reasons.

After the Cold War ended, Democrats “coalesced around the use of American power to prevent genocide and other gross violations of human rights,” William A. Galston, a senior fellow at the Brookings Institution who was an adviser to President Bill Clinton, wrote this week. “But for many of today’s Democrats, Iraq serves as the moral equivalent of Vietnam and evokes comparable doubts about the use of American power.”

Democrats say that Assad’s alleged use of chemical weapons should not go unaddressed, but they also are haunted by the war in Iraq. In 2002, Congress gave President George W. Bush broad authority to invade. The resolution, based on faulty intelligence that Iraq had stockpiled weapons of mass destruction, opened the door for an unpopular conflict that lasted nearly nine years.

“For my constituents, it’s not overshadowing — it’s at the core of their concerns, misgivings, doubts,” said Rep. George Miller (D-Calif.). “It gnaws right at you.”

Miller, who was elected to the House in 1974 as the Vietnam War was ending, said that the skepticism in his district is “as intense as I’ve ever seen it.”

For Republicans, the Syria debate has exacerbated decades-old rifts between those who favor more international involvement, such as McCain, and a newly rising libertarian, anti-interventionist wing led by Paul.

In recent days, it appears that more Republicans, particularly those with future political aspirations, are siding with Paul on Syria, even if they are simultaneously seeking out some sort of middle ground between the sharp-tongued senator from Kentucky and the hawkish McCain.

Several Republican strategists described the Syria vote as a dilemma for most GOP lawmakers — a “rat’s nest,” as anti-tax activist Grover Norquist put it — who are figuring out a coherent modern-day foreign policy.

The vote could be a defining moment for some of the rising stars in the party, who have had the luxury of having it both ways until now.

Rubio, for instance, voted against the Syria strike resolution during a Senate Foreign Relations Committee meeting this week, but followed his vote with a speech blasting those in his party who oppose U.S. intervention in the world.

“It’ll be an interesting test of where is the Republican Party on these issues of getting involved in a foreign war absent an attack on the United States,” Norquist said. “If you’re Rand Paul, you’re standing on principle. But if you’re Ted Cruz, will you be seen as playing politics?”

Cruz, a freshman GOP senator from Texas who is a favorite of the tea party and a possible 2016 presidential contender, has not said which way he plans to vote. After a classified briefing with administration officials this week, he said that he was “deeply skeptical” of the administration’s policy toward Syria.

“Inserting the United States military into a sectarian civil war in Syria is profoundly perilous,” Cruz said. “To assume this risk, we must be confident the potential national security benefits of such a mission outweigh the risks.”

The dynamic may well change when Congress returns to Washington next week. Rep. David E. Price (D-N.C.) said he is looking forward to an opportunity to read the intelligence on which Obama is basing his belief that Assad’s government launched the Aug. 21 chemical weapons attack that U.S. intelligence officials say killed 1,429 people.

But, Price said, “there is historical experience here, after all, that needs to be remembered. There is just an aversion to another Middle East war.”