(Reuters) - 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...