The controversy has split the network of more than 930 food banks across the country that, like the one in Essen, belong to a charity called the Tafel. The charity has grown to 60,000 volunteers and serves 1.5 million people across Germany. Many of them have experienced similar tensions.

Sabine Werth, who now runs the Berlin subsidiary, founded the network in 1993, when a wave of homelessness swept across her city. “One of our founding principles is that we serve according to need, not origin,” said Ms. Werth, 61.

What Mr. Sartor has done, she said, amounts to “Germany First.”

But Germany First is popular with many, as Ms. Werth has learned the hard way in recent days. “Cockroach,” “piece of dirt” and “foreigner’s slut” are some of the insults that have landed in her inbox. One longtime donor diverted his donation from Berlin to Essen, she said.

He is not the only one. Mr. Sartor proudly showed off his donation account: Over the past two weeks the food bank has received as much as it would normally raise in six months. Some try to earmark their donation to Germans only, but Mr. Sartor does not accept those.

His inbox is mostly full of praise: “Keep going” one message read. “God bless you,” said another. He has 2,340 unread emails.

The nationwide head of the charity, Jochen Brühl, said the debate currently animating the country was largely missing the point. Germany is Europe’s richest country and has a budget surplus of more than 40 billion euros ($55 billion), he pointed out.

“The whole country is up in arms about this one little food bank in Essen,” he said, “when the real scandal is that in this rich country we have this kind of poverty.”