The result has been long in coming, and predictable. Democracy has pushed back, through bottom-up agitation from ordinary people who had taken to the streets from Leipzig to Bucharest. And when the elites pushed back, whether it was over the Greek debt crisis or the refugee influx, who was standing alongside them? The erstwhile voice of the people, the Social Democrats.

This is not to indict the European center left; who can forget the flush of post-Communist optimism? Democracy’s thorough victory in the Cold War appeared to prove that it was self-supporting. But that optimism led to carelessness, and the naïve belief that labor and capital could finally unite under the flag of Davos.

Nor was this completely wrong; the world is a much better place thanks to freer markets and political integration. The problem is that these benefits have been dwarfed by the trickle-up effects to a fortunate few. The increasing wealth has been spread unfairly, and not just in terms of money: Risk of failure has been handed downward to the citizens. This became ultimately apparent in the wake of the financial and the euro crises. It was the regular European who eventually had to pay for high-flying dreams of the Davos elite.

Instead of at last addressing this contradiction, prominent Social Democrats appear intent on solving the dilemma of internationalism by making it bigger. Martin Schulz, the party’s leader, has just proposed a “United States of Europe” by 2025 — and to expel European Union member states that won’t join in his flight of fancy.

Social democracy’s second contradiction is that you can’t promote a borderless world and the welfare state at the same time. Ms. Merkel’s predecessor, the Social Democrat Gerhard Schröder, recognized this and pruned back Germany’s overgenerous welfare spending to make the economy more competitive. It was the right move, however unpopular, and the party wisely defends it today. But following these welfare restrictions, how can you convey to your electorate that you are opening the doors of your country to a million refugees and migrants who are entitled to welfare payments?

In the last election, millions of angry left-wing voters in places like the Ruhrgebiet, Germany’s equivalent of the Rust Belt in America, saw a state that had plenty of money for others but not for them. The response of Social Democrat leaders was to label such critics “right-wingers” and to demand they embrace even more liberal virtues, like identity and gender politics. This prompted a defection of hundreds of thousands of Social Democratic voters to the far-right Alternative for Germany, which scored almost 13 percent in the election.

Sigmar Gabriel, who preceded Mr. Schulz as the head of the Social Democrats and who remains vice chancellor until a new government is formed, seems to get this. In an essay for Der Spiegel, he wrote that the plight of the Democrats in the United States shows “how dangerous it is to focus on issues of postmodernism.” He added, “If one loses the workers of the Rust Belt, the hipsters in California aren’t going to be of any great use either” in winning national elections.

It’s not clear his party gets it, though. With no idea how to appeal to both Berlin hipsters and industrial workers in the Ruhrgebiet, it is no surprise the Social Democrats lack the courage to dig down to the root of their misery. As long as they avoid the two fundamental contradictions of modern social democracy, the decline will continue. And rightly so.