By Thomas Ferraro and Richard Cowan
WASHINGTON (Reuters) - The fate of a congressional resolution to authorize President Barack Obama's planned military strikes on Syria hinged on Thursday on scores of undecided U.S. lawmakers, with party loyalty appearing increasingly irrelevant.
Even after congressional hearings featuring Obama's secretaries of state and defense, a half dozen closed-door briefings and phone calls from Obama himself, it was too close to call on whether Congress will authorize military force.
Obama asked Congress to back his plan for limited strikes in response to a chemical weapons attack on civilians that the United States blames on Syrian President Bashar al-Assad's forces.
First-term Democratic Senator Joe Manchin, who had been seen as a possible swing vote, dealt the president a setback when he announced on Thursday he would oppose the resolution to authorize military strikes.
"Given the case that has been presented to me, I believe that a military strike against Syria at this time is the wrong course of action," Manchin said.
Republican Representative Michael Grimm, who initially backed Obama's call last month for military strikes, withdrew his support on Thursday. "Unfortunately, the time to act was then and the moment to show our strength has passed," said Grimm, a Marine combat veteran.
If Obama fails to win congressional support, he would face two undesirable options. One would be to go ahead with military strikes anyway, which could provoke an angry showdown with Congress over their respective powers.
The other would be to do nothing, which White House officials privately acknowledge would damage the credibility of any future Obama ultimatum to other countries.
Twenty-four of the Senate's 100 members oppose or lean toward opposing authorizing military strikes, according to estimates by several news organizations, with an equal number favoring military action and roughly 50 undecided.
Every vote will count in the Senate, where a super-majority of 60 will likely be needed because of possible procedural hurdles for a final vote on approving military action.
A count by the Washington Post listed 103 members of the House of Representatives as undecided, of whom 62 are Democrats. There are 433 members currently sitting in the House.
Party loyalty, which drives most issues in a Congress known for its partisan gridlock, was becoming increasingly irrelevant, particularly among Obama's fellow Democrats. Some Democratic liberals who usually line up behind Obama's policies have expressed reluctance to back an attack on Syria.
'I'M AN ADULT'
"I support the president," said Democratic Representative Bill Pascrell, who remained undecided.
"I want him to succeed. But he isn't asking me to be - nor will I be - a lap dog. So I will make my own decision. I'm an adult," Pascrell said.
Republicans have opposed Obama on a host of issues in Congress - and those aligned with the conservative Tea Party movement appear likely to do so on this matter. But other Republicans who favor strong American engagement internationally are lining up behind the Syria military strike authorization.
Most House Republicans are expected to vote "no," even though their top two leaders, Speaker John Boehner and Majority Leader Eric Cantor, have endorsed the military strikes.
While Obama administration officials continued to express confidence about ultimately winning congressional support, it was clear on Thursday that their blitz of briefings was not having the desired impact, especially with many lawmakers reporting opposition to strikes among their constituents.
Manchin said he listened to the concerns of thousands of people in his home state of West Virginia, attended hearings and briefings, and spoke with former and current military leaders.
In a statement, he said that "in good conscience, I cannot support" the resolution authorizing force and that he will work to develop other options. "I believe that we must exhaust all diplomatic options and have a comprehensive plan for international involvement before we act," Manchin added.
Democratic Senator Barbara Mikulski told reporters, "I have more questions than I have answers, and I hope to get them over the course of today and tomorrow."
She spoke as she entered the latest closed-door session on Thursday with Obama's national security team, only to emerge two hours later saying she still had "more questions."
"What we heard today made a compelling forensic case that, one, nerve gas was used, and number two, that it was used" by Assad's forces, Mikulski said. "The next step, then, has to be ... what is the way to both deter and degrade his ability to ever do it again? ... Does a military strike do that?"
FIRST HURDLE CLEARED
The Democratic-controlled Senate and Republican-controlled House both must approve the measure. It cleared its first hurdle on Wednesday when the Senate Foreign Relations Committee approved the resolution by a 10-7 vote - with Democrats and Republicans voting on both sides of the issue.
The full Senate is likely to begin voting next Wednesday, a Senate aide said. It will start with a vote on an anticipated legislative roadblock by Republicans, and then move on to a vote on the resolution to authorize the use of force, the aide added.
The timing of a vote in the House remained unclear.
Memories of the protracted wars in Iraq and Afghanistan are still fresh in the minds of members of Congress, leaving many in both parties worried that a military strike could lead to a longer and larger U.S. engagement in Syria.
If Obama is going to win passage of the measure in the House, he must convince fellow Democrats like Representative Zoe Lofgren and Pascrell.
The two liberals have been reliable Obama allies on a crush of issues since Obama entered office, but now voice plenty of questions and concerns about his bid to attack Syria.
Lofgren joined a conference call for House Democrats on Monday given by Obama administration officials. Lofgren complained that the briefing did not provide nearly as much information as she had sought and disliked at least a portion of Secretary of State John Kerry's presentation.
Kerry invoked memories of Nazi Germany when he told the House Democrats that the United States faces "a Munich moment" in deciding whether to wage military strikes against Syria.
"I thought it was a very unfortunate comment. We need facts, not overheated emotional rhetoric," Lofgren said.
(Additional reporting by Susan Cornwell, Susan Heavey and Rachelle Younglai; Editing by Fred Barbash and Will Dunham)