BOCHOLT, Germany—Martin Schulz, the Social Democratic candidate for chancellor of Germany, worked up a crowd at a rally this week by recalling his party’s defiant opposition to the Nazis and exhorting them to confront what he said was another malign force: U.S. President Donald Trump.

Mr. Trump’s “attacks on Europe are also attacks on Germany,” Mr. Schulz said. “In a time when the world is drifting apart, in a time of Trumpism, we need values-based cooperation of the democracies in Europe now more than ever.”

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