WASHINGTON—Senior Obama administration officials have caught some lawmakers and allies by surprise in recent weeks with an amended approach to Syria: They don't want an outright rebel military victory right now because they believe, in the words of one senior official, that the "good guys" may not come out on top.

Administration officials fear that with Islamists tied to al Qaeda increasingly dominating the opposition to Syrian President Bashar al-Assad, too swift a rebel victory would undercut hopes for finding...