CARACAS, Venezuela — After more than a decade in power, Venezuela’s strongman, President Nicolás Maduro, is facing the greatest threat yet to his authoritarian rule.

       On Sunday, Venezuelans will choose a leader in an election that Maduro pledged would be free and fair. The opposition, whose candidate leads in polls by double-digit percentage points, is hoping for a victory so overwhelming that it will compel Maduro to negotiate a peaceful handover — and a transition away from the socialist government founded by Hugo Chávez a quarter-century ago.