PARIS — A struggling French left named its contender for the presidency Sunday, in the midst of a highly contentious election campaign that is likely to shape the future of a deeply troubled Europe.

The winner was Benoît Hamon, a onetime education minister under François Hollande, the historically unpopular Socialist president whose unusual decision not to seek reelection led the way to Sunday’s leftist primary. Hamon defeated Manuel Valls, Hollande’s prime minister, with roughly 60 percent of the vote.

The newly anointed winner promised to unite the warring factions of the French left in advance of the elections in April and May. “France needs a left that thinks of the world as it is,” he said.

Given Hollande’s unpopularity, many were quick to explain the outcome as a rejection of the sitting president, who has struggled with stagnant unemployment figures and terrorist attacks, which claimed the lives of 230 people in France over the past two years.

But analysts saw Hamon’s victory as something far more seismic: the likely demise of the French left, and specifically the Socialist Party, as a force to be reckoned with in French and European politics.