WASHINGTON – After days of setbacks in St. Petersburg and Washington, where US President Barack Obama has tried to recruit support for a strike against Syria’s reigning President Bashar Assad, the White House is preparing for one of its most important weeks in five years.

Congress reconvenes on Monday to debate and vote on authorizing the use of force.

Washington won key support from the European Union after the conclusion of the G20 summit on Friday, hosted this year by Russia, whose leader, Vladimir Putin, vowed last week to aid Assad militarily if the US attacks his assets.

In a carefully worded statement, the EU said that evidence clearly showed the Assad regime carried out a chemical attack against civilians on August 21 on the outskirts of Damascus, killing more than 1,400 people, including hundreds of children.

But the statement emphasized the need for a global response that respected the procedures of international bodies, slighting Obama’s efforts to circumvent the United Nations Security Council, where Russia holds veto power.

To make his case to a wary public, Obama is to address the US from the White House on Syria on Tuesday, a day before the Senate is scheduled to vote on the resolution. The House of Representatives would then take up its own version of the resolution.

But each day since Obama decided to seek congressional approval to strike Syria, he has lost a handful of votes. More than 120 representatives have already declared their opposition to the measure. The resolution will be defeated if 217 votes are cast against it in the House of Representatives.

For the first time since Obama took office in January 2009, the White House has called on the American Israel Public Affairs Committee to lobby on its behalf on Capitol Hill.

AIPAC will send more than 250 activists and lobbyists to Congress once its members reconvene on Monday, in an effort to “flood the zone” with support, officials said.

Pro-Israel US groups had largely kept a low profile on the Syria issue, while acknowledging they had made it known that they supported American action, concerned about instability in Syria and what message inaction might send to Assad’s ally, Iran.

But they generally wanted the debate to focus on US national security rather than how a decision to attack Syria might help Israel, a reflection of their sensitivity to be seen as rooting for the US to go to war.

Before Congress left for summer recess at the beginning of August, the House of Representatives passed a harsh sanctions bill against Iran that could bring its oil exports down to zero if it doesn’t capitulate on its nuclear program. That resolution passed 400 to 20.

AIPAC pushed aggressively for its passage.

But to win the vote on Syria, the White House finds itself trying to appease members of Congress who often voice competing concerns.

Some members are worried that the planned attack will do more harm than good, emboldening Assad and embroiling the US and its allies in another Middle East war with “mission creep” – or the slow expansion of objectives beyond the original plan.

Others worry that the plan of attack is not forceful enough.

To allay the concerns of those worried about a broader mission, the White House has accepted draft language for the resolution that limits its scope, including a time limit of 60 days, with a possible 30-day extension, and an explicit provision that prohibits US boots on the ground.

Crisis in Syria - full JPost.com coverage

And to appease those concerned that the operation – as has been described by Obama’s cabinet – will be mostly symbolic, the White House has strategically leaked plans for a strike much more extensive than originally thought.

In addition to the more than 200 Tomahawk cruise missiles that can be fired from American warships in the Mediterranean, the Pentagon is preparing to send B-52 and B-2 bombers from US territory to drop bombs with significant payloads, according to US officials.

Gen. Martin Dempsey, chairman of the US Joint Chiefs of Staff, said that any equipment that could be used to deliver, stock or protect chemical arms could be targeted, including air defenses, airports and longrange missile caches.

“This military strike will do more damage to Assad’s forces in 48 hours than the Syrian rebels have done in two years,” one national security official told ABC News.

In St. Petersburg, Obama said he was not “itching” for military action against Syria, and that he was open to “constructive” ideas from Congress that would delay an attack such as offering Assad a fixed window to sign the Chemical Weapons Convention and forfeit his 1,000-ton stockpile of chemical arms.

But gearing up for an attack many see as inevitable, Assad has reportedly planned to strike back against American facilities and interests in the region along with Iran, according to US officials.

The Wall Street Journal quoted officials as saying the delay in a US strike against Syria increases the chance for the Shi’ite gunmen in Iraq and other allies of the Assad regime to coordinate attacks against US interests.

US military forces in the region are preparing for retaliation by Iranian vessels in the Persian Gulf against American warships, according to the report.

Hezbollah could target the US Embassy in Beirut. The State Department evacuated the facility of all nonessential personnel on Friday and issued a travel warning for all of Lebanon.

Syrian and Iranian officials, as well as Hezbollah, have warned repeatedly that retaliation for a US strike against the Assad regime would be directed against Israel. Iran’s Revolutionary Guards chief, Maj.- Gen. Muhammad Ali Jafari, said a week ago that a US military attack on Syria would lead to the “imminent destruction” of Israel and would prove a “second Vietnam” for America.

Reuters contributed to this report.

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