Europe’s social democratic parties are collapsing — and their leaders don’t seem to know how to reverse the trend. For much of the 20th century, center-left parties rooted in trade unions and industrial labor were among Europe’s dominant political forces. But today, many of them are politically unrecognizable — or in dire straits. The latest example is Danish Prime Minister Mette Frederiksen’s Social Democrats, which last week suffered a dramatic slump in national elections. Although the party