It seems that everyone in Washington is talking about it except President Barack Obama: When Congress votes on the administration's request to use military force in Syria, the future of his presidency could well be on the line.

A defeat, a distinct possibility, would hobble Obama in affairs both foreign and domestic, particularly if fellow Democrats collaborate in it.

It will hurt him at a critical juncture, as he confronts not only Syria, but the nuclear activities of Iran and North Korea, another round of battles with Republicans over fiscal issues, an immigration bill, and a possibly difficult nomination fight over a new chairman of the Federal Reserve.

Using Obama's presidency as an argument as Congress ponders a resolution authorizing military action is off-limits for the administration - it would make the debate about Obama and cost the president votes from some Republicans he is counting on.

"My credibility is not on the line," Obama said at a news conference in Stockholm on Wednesday, five days after he announced he would seek congressional authorization for a strike on Syria over an August 21 chemical weapons attack in that country.

"The international community's credibility is on the line. And America and Congress' credibility is on the line."

But if ever there was an "elephant" in a room, the Obama legacy is it.

A 'no' vote would be a "catastrophe" for Obama, said David Rothkopf, a former Clinton administration official who is now president of Garten Rothkopf, an international advisory firm.

"It would ratify the perception of him as a lame duck at one of the earliest points in recent presidential memory," Rothkopf said. "He would appear to be weakened and unlikely to get much done during the remainder of his term."

"I think a 'no' vote would be a huge slap at the president," said George Edwards, a presidential scholar at Texas A&M University. "It would seem to tie his hands."

It would hurt Obama even more if many Democrats - members of his own party - vote against him, which at the moment seems likely.

House of Representatives Speaker John Boehner in particular knows the consequences of being a leader with a diminished following. During the fiscal cliff confrontation in December, his fellow Republicans in the House defeated a proposal he thought might help resolve the fight over tax increases and heavy automatic spending cuts.

Boehner has since taken a back seat in confrontations with Obama in part because he can no longer speak for his caucus in the House.

Obama will confront a difficult challenge in October, when he faces Republican demands to make spending cuts in exchange for an increase in the nation's borrowing limit, the debt ceiling.

He faces another potential fight if he nominates Larry Summers, said to be his current favorite to replace Ben Bernanke at the Fed. Bernanke's term ends January 31 and the White House has said an announcement on his successor is expected in the autumn.

At stake domestically in the Syria vote is the president's "political capital," the influence that presidents gain with every victory and lose with every defeat, particularly if they have been personally engaged in the issue.

Political capital is unquantifiable, and the impact on domestic issues a matter of speculation. The significance of defeat for Obama in the international sphere, beyond Syria, is more clear.

Indeed, for Obama and his national security team, the vote on Syria appears to represent a desire to get a clearer fix on whether they can count on Congress if the Iranian nuclear standoff comes to a head or North Korea escalates its provocations to new levels, a US official said.

A congressional "no" vote this time around would weigh heavily against seeking congressional approval should Obama feel the need to use force again.

Without reference to Obama, Secretary of State John Kerry has painted a dire picture in hearings on the resolution. A 'no' vote from lawmakers, he and other officials have argued, would embolden Iran and North Korea and make it more likely that a terrorist group might use illicit weapons.

No one doubts that Republicans would use a defeat to their advantage. A central Republican critique of Obama is that he is a weak leader. A 'no' vote on Syria delivered in part by Democratic lawmakers would strengthen their argument, just as the administration is preparing for the fiscal battles of the autumn.

While the administration has sought to divorce the issue from Obama personally, the stakes for his presidency are on the minds of many Democrats in Congress as they consider their votes.

"Sure, you weigh that," California Democratic Representative George Miller, who has not decided which way to vote, told Reuters in an interview. "You obviously weigh that, but that cannot be the determining factor . ... Obviously, I want the president to succeed."

But "when I run into my constituents," Miller added, "I've been asking them their opinion. They are very, very deeply concerned about any involvement by us there."-Reuters