The twists and turns in the Syria debate have whipsawed the nation’s capital and by some accounts imperiled Mr. Obama’s presidency. Democrats are mystified and in some cases livid with Mr. Obama for asking Congress to decide the matter instead of simply ordering one or two days of strikes and getting it over with.

By most estimates, the Republican-controlled House would reject authorizing such an attack if the vote were held now, and it is not clear whether the Democrat-led Senate would approve it. Few presidents have lost such a major vote on war and peace in the almost century since the Senate rejected Woodrow Wilson’s League of Nations.

In their private moments, Mr. Obama’s allies said even the argument that his presidency would for all intents and purposes be over did not sway some unsympathetic Democrats, frustrated over how few victories there have been to hang on to in Mr. Obama’s fifth year in office.