To be sure, Mr. Trump’s choice of words drew condemnation from around the world. Botswana and Haiti asked for meetings with American diplomats to clarify what Mr. Trump said and what he believes. The president of Senegal, Macky Sall, was one of many who saw racism in the remarks. “Africa and black people deserve the respect and consideration of all,” he wrote on Twitter.
Even the Vatican newspaper, L’Osservatore Romano, weighed in, declaring Mr. Trump’s comments “particularly harsh and offensive.”
But the political reality is that migration has become a salient issue — and not only for right-wing, populist and nativist politicians. Across many affluent societies, people are anxious about technological change, rising inequality and stagnant wages, and they have focused their ire at the global flows of capital and, especially, labor. There are also concerns about demographic change, as the world becomes less white and as western societies age.
Moreover, the chaos and violence that have driven people from the Middle East, Southwest Asia and sub-Saharan Africa to seek to live elsewhere, even as far away as Australia and Canada, have also raised fears about refugees who do not appreciate the values of the countries hosting them — or even worse, fears of terrorists taking advantage of humanitarian policies to infiltrate societies and then carry out attacks.
Hours before the news of Mr. Trump’s comments broke on Thursday, the United Nations secretary general, António Guterres, spoke about the need for more “safe, orderly and regular” migration.
He implored nations to “use facts, not prejudice”ess the challenges of migration.
“Globally, migration remains poorly managed,” he acknowledged. “The impact can be seen in the humanitarian crises affecting people on the move, and in the human rights violations suffered by those living in slavery or enduring degrading working conditions. It can be seen, too, in the political impact of public perception that wrongly sees migration as out of control. The consequences include increased mistrust and policies aimed more at stopping than facilitating human movement.”
Mr. Knaus, of the European Stability Initiative, was one of several commentators who expressed fear that Mr. Trump would only embolden xenophobic rhetoric.
“This will have consequences because it’s widely followed,” he said. “In every Austrian and German village, they follow what the U.S. president is doing.”
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.
Prominent among them was Viktor Orban, the prime minister of Hungary, who criticized non-Christian migrants and then built a wall to stop migrants from entering Hungary.
European leaders initially distanced themselves from him — in both word and action. But by March 2016, several had come around to his policy suggestions, if not the tone in which he had made them.
While still moderate in tone, some leaders are pursuing policies that are Trumpian in spirit, said Catherine Woollard, secretary general of the European Council on Refugees and Exiles, an alliance of nearly 100 refugee rights groups.
“There’s a denial among many European leaders that they’re anything like Trump — while they promote measures that will have the same impact” as Mr. Trump’s restrictions, she said.
Italy’s relationship with Libyan militias, in which the country provides inducements for Libyans not to send migrants to Europe by boat, is among the policies that has concerned advocates for refugees.
Though the policy has been justified in humanitarian terms, because it has reduced the number of people put at risk in the Mediterranean, it has also trapped thousands of migrants in slavery-like conditions on Libyan soil.
In Brussels, European Union leaders are debating whether and how to change the bloc’s common asylum policy. Central and Eastern European countries are among those that support changes Ms. Woollard believes will curb refugee rights in Europe. But so, too, is Germany, widely seen as one of the countries most sympathetic to refugees.
Then there is a contentious deal between the European Union and Turkey that is aimed at closing the main migration routes into Eastern Europe. European Union leadership has justified this as a humanitarian gesture, since it has curbed the number of migrant shipwrecks.
But Ms. Woollard feels this justification has “sugarcoated” the actions of Western European leaders, as it has also confined thousands of asylum seekers in impoverished conditions in Greece and Turkey.
Restrictive migration policy is “not solely confined to the black sheep in the East,” Ms. Woollard said.
Mr. Knaus, who first dreamed up the parameters of the controversial E.U.-Turkey agreement, said it was wrong to conflate the racist rhetoric of Mr. Trump and Mr. Orban with efforts by less reactionary leaders to exert control over migration.
“There is a danger that in opposing this kind of rhetoric one basically strengthens it because we say that anyone who is in favor of controlling borders is in favor of Trump,” Mr. Knaus said. “That is very dangerous because it concedes to racist politicians that being in favor of control is the same as being racist. And it’s not.”