Since then, Mr. Trump has warned that he will “definitely” slap Europe with tariffs “pretty soon,” raising the same wariness that many Europeans feel among those on the right who would seem to be his natural allies.

While Mr. Trump promises to answer to no one as he prioritizes American interests, many nationalist parties in Europe pledge to do the same for their own countries. Mr. Trump’s threats go to the heart of their own agendas, and they could hurt the core constituencies on which nationalist parties have expanded their appeal.

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