UNITED NATIONS — By vetoing a Security Council resolution condemning Syria for its oppression of antigovernment forces, Russia and China effectively tossed a life preserver to President Bashar al-Assad, seemingly unwilling to see a pivotal ally and once stalwart member of the socialist bloc sink beneath the waves of the Arab Spring.

A double veto at the United Nations is rare, in this case driven by similar if not exactly parallel concerns in Moscow and Beijing about losing influence in the Arab world as one authoritarian government after another built on the now-faded Soviet model collapses.

“They are gambling that Assad can hold on now; it seems to be an expression of confidence that he can cling to power,” said Fiona Hill, a Russia expert at the Brookings Institution.

Russia enjoys military and commercial deals with Syria worth billions of dollars annually, plus its alliance and only reliable Arab friend give it an entree into the Arab-Israeli peace negotiations. In addition, Moscow maintains perks left over from its superpower days, for instance, a naval base at Tartus, Syria, that accommodates visits by warships like Peter the Great, a nuclear-powered missile cruiser, during its Mediterranean jaunts.