In Western Europe, heads of government have sometimes described migrants in terms of floods and hordes — but the most abusive language has usually been restricted to far-right opposition politicians like Marine Le Pen in France and Geert Wilders in Holland.

“What is dangerous is if this kind of language migrates from the far-right to the mainstream,” Mr. Knaus added.

But some in migration policy circles fear this transition has already begun.

Several European heads of government were proudly xenophobic in their responses to a refugee crisis in 2015, when more than one million asylum seekers arrived by boat on European shores, prompting a surge in support for far-right parties and nativist rhetoric — particularly in Central and Eastern Europe.