LONDON — The British Labour Party is in meltdown. After reviving the center-left in the 1990s, and then dominating British politics until 2010, Labour now faces the gravest challenge in its 116-year history. One of the oldest social-democratic parties in the world is fighting to survive; there is no guarantee it will.

Labour’s crisis is a microcosm of the test that confronts social democracy at large. In polling, Labour has fallen to its lowest level in generations. Shortly before Christmas, the party’s support dropped to 24 percent, which if repeated in a national election would mark its lowest share of the vote since 1918. Forecasts suggest that the number of Labour seats in Parliament could slip from the 232 the party won in 2015 to 190 in 2020, its poorest showing since 1935.

The bad news does not end there. In a recent election to fill a vacant parliamentary seat, Labour suffered the humiliation of failing to reach even 5 percent of the vote. In Scotland, where Labour lost all but one of its seats in 2015, leaked internal polling not only put the party a distant third behind the Scottish nationalists and the Conservatives, but also offered this ominous warning: “There is no such thing as a core Labour vote anymore.”

This week, a report from the center-left Fabian Society suggested that the Labour vote could fall in the 2020 general election to as low as 20 percent because in previous elections it has underperformed its midterm polling by an average of 8 points. This would leave the main opposition party with only 140 to 150 seats in Parliament. The report spoke “of insignificance, even of looming death.”