Then came the Great Recession. With the old guard spent or discredited, and voters longing for a new start, those traditional power relations were upended. The first sign of the new era came in January 2015 when Syriza, a party forged from a potpourri of leftist splinter groups, won national elections at the height of Greece’s devastating currency crisis. The party’s young leader, Alexis Tsipras, became the first far-left politician in decades to head a western European government.

That summer, Corbyn took over Britain’s Labour Party on the promise of breaking with the “neoliberal” policies of his predecessors. By the end of the year, Pablo Iglesias, a young Marxist academic, had succeeded in turning an inchoate protest movement into one of Spain’s biggest parties. And when Emmanuel Macron’s election obliterated France’s traditional parties in the spring of 2017, Mélenchon, a hard-liner with close connections to a variety of communist factions, became the de facto leader of that country’s left.

These upsets seemed to demonstrate that the far left had greater electoral potential than previously recognized. But in the excitement, many observers failed to absorb that these victories mostly consisted of a reordering of power within the left, rather than a triumph over the right. Even in Greece, the one case where the left did manage to win a general election, it needed the support of a far-right populist party to form a government. A big question thus hung over the success of these new leaders: Would they be able to retain the loyalty of their most ardent fans, and expand the ranks of their supporters, once the public got to know them better?

The first serious sign of trouble came in late May, when elections for the European Parliament provided a snapshot of the far left’s standing across the continent. In Spain, Podemos, down to 10 percent of the vote, was eclipsed by the PSOE, its center-left competitor. In France, Mélenchon sank to 6 percent. Other far-left parties in countries from Germany to Italy posted similarly disappointing results.

Corbyn, the European leader most heralded as a harbinger of the future by leftist cheerleaders in the United States, has met with an even more radical reversal: He now ranks as one of Britain’s least popular politicians. The Liberal Democrats, a centrist party that had once looked mortally wounded by its participation in an unpopular coalition with the Conservatives, beat Labour in the European elections, and might just be able to repeat that performance in the next general elections. One recent poll even raised the possibility that Corbyn could then lose his own seat, a constituency in central London that Labour has held since before World War II.

Sunday’s elections in Greece provide the strongest indication to date that the left is now in deep crisis: Less than four years after he took office, Tsipras has been swept aside by New Democracy, the center-right party that has governed the country for much of the past 40 years.