(Recasts, adds McCain, Graham comments)

By Steve Holland and Susan Cornwell

WASHINGTON, Sept 2 (Reuters) - President Barack Obama's efforts to convince Congress to back his plan to attack Syria met with skepticism on Monday from lawmakers in his own Democratic Party who expressed concern the United States would be dragged into a new Middle East conflict.

Obama appeared to make some headway, however, with two influential Republican senators, John McCain and Lindsey Graham, who came out of a White House meeting with him convinced that the president is willing to do more than simply fire off cruise missiles and also wants to bolster the Syrian opposition.

McCain, long an advocate of a more robust U.S. approach to Syria, said failure to get behind strikes against President Bashar al-Assad would be "catastrophic."

Obama's abrupt decision to halt plans for a strike against Assad's forces and instead wait for congressional approval has generated a raging debate just as the president prepares to go to Sweden and Russia this week.

Armed with evidence they say proves Syria's government killed over 1,400 people with nerve agent sarin, Obama's top national security aides made their case to Democratic members of the U.S. House of Representatives in a 70-minute conference call, urging them to back Obama's request.

The White House argument is that Syria must be punished for the Aug. 21 chemical weapons onslaught and that at stake is the integrity of an international ban on such weapons and the need to safeguard U.S. national security interests and allies Israel, Jordan and Turkey.

Syria has blamed the attack on rebel forces.

As with much in which the divided Congress involves itself, there was deep disagreement on how to proceed, with some lawmakers worried the United States might be drawn into yet another Middle Eastern conflict in spite of Obama's pledges for a limited strike.

Overshadowing the debate are the ghosts of wars in Iraq and Afghanistan, conflicts that lasted far longer and were far more expensive than first predicted. Congressional hesitancy reflects the overall weariness of war among Americans, who oppose getting involved in Syria.

Representative Chris Van Hollen, a senior Democrat and an ally of Obama on many issues, complained that the wording of the White House's request to Congress for the authorization of the use of force was too open-ended and could lead to deep U.S. involvement in Syria, where more than 100,000 people have died in more than two years of conflict.

"There is no limitation on putting American soldiers on the ground. There is no end point" on the resolution, he said. "The draft resolution presented by the administration is overly broad, it provides too much of a blank check to the executive," he said.

Van Hollen was among Democratic lawmakers briefed by conference call by Secretary of State John Kerry, Defense Secretary Chuck Hagel, national security adviser Susan Rice, intelligence director James Clapper and General Martin Dempsey, chairman of the Joint Chiefs of Staff.


OPPOSITION IN BOTH PARTIES

Obama needs to shore up his left wing, particularly in the House, where liberal Democrats and many conservative "Tea Party" Republicans are both opposed to more U.S. military action in the Middle East.

McCain and Graham are from the more traditional wing of the Republican Party that generally favors U.S. intervention abroad when deemed necessary. They want a broad strategy not just to punish Assad for the chemical weapons attack but to help Syria's rebels.

The Democratic-led Senate is expected to approve U.S. military action, but failing to win the support of McCain and Graham would be a blow to Obama's wider influence over Congress on Syria.

"I guess the way I would term the conversation is that there is a consensus being formed that we need to degrade Assad's capabilities and upgrade the Syrian opposition," Graham told reporters when he and McCain emerged from a one-hour White House meeting with Obama.

Representative James McGovern, a liberal Democrat from Massachusetts, said he was skeptical that Obama's plan for military strikes could help end the war in Syria. If the vote were taken today, he would vote no.

"People are horrified by the pictures of people suffering and they genuinely want to help. But people have become, it's more than just war-weary, they've become skeptical of the effectiveness of these military involvements," he said.

With Navy ships in place and ready to launch cruise missiles on Obama's order, no decision was likely until days after Congress returns from its summer recess on Sept. 9. In the interim, Obama is using the time to build his case.

Washington's hesitancy has prompted mocking comments from Syrian leaders and a push from Assad's chief backer, Russia, to send members of the Russian parliament to the U.S. capital to argue against a strike.

Obama's gamble to seek congressional backing carries many risks, chief among them is that Congress will again thwart him and make him look weak around the world.

It may depend on building a majority House vote based on Obama's fellow Democrats joining those Republicans who support action, a senior House Republican aide told Reuters on Sunday.

"It's too early to speculate" what the House will do, the aide said, "but (House Democratic leader Nancy Pelosi) is going to have to post a big number" among her members in support of it. The aide declined to predict how big.

Republicans hold the House, 233-200, with two vacancies. (Additional reporting by Jeff Mason, Patricia Zengerle and Thomas Ferraro; Editing by Jackie Frank and Peter Cooney)

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