WASHINGTON — President Obama, facing implacable opposition to a strike against Syria in Congress and throughout the country, said Tuesday that he would hold off on military action for now and pursue a Russian proposal for international monitors to take over and destroy Syria’s arsenal of chemical weapons.

Speaking to the nation from the White House, Mr. Obama laid out his most extensive and detailed case for an attack to punish Syria for its use of chemical weapons. But he also acknowledged the deep doubts of Americans who after the experience of Iraq and Afghanistan view any form of military engagement in Syria with alarm.

In a speech that only 48 hours ago was going to be solely a call to arms, Mr. Obama instead offered a qualified endorsement of a proposal that his own advisers conceded was rife with risk, given Russia’s steadfast refusal to agree to any previous measures to pressure Syria, its longtime ally.

“It’s too early to tell whether this offer will succeed, and any agreement must verify that the Assad regime keeps its commitments,” Mr. Obama said. “But this initiative has the potential to remove the threat of chemical weapons without the use of force.”