CARACAS, Venezuela — It hardly seemed like a victory celebration. Nicolás Maduro stepped onto a stage here Sunday night after being declared the winner by a narrow margin in the presidential election to replace his mentor,Hugo Chávez, but his supporters were already streaming away in droves. The long faces told the story. “It’s not the same coming here and not seeing Chávez,” said Octavio Fuentes, 35, a government worker, walking out early. “We won, but you can’t compare Maduro with Chávez.” Mr. Maduro was supposed to ride Mr. Chávez’s immense popularity and the wave of mourning over his death last month to a resounding victory that would ratify the leader’s idiosyncratic revolution.