BRUSSELS — The European stage Angela Merkel commanded for so long and so effectively may be cracking, if not collapsing. She has been the poster woman for Europe’s democratic center, but the center is imploding. She and Germany have been a symbol of stability, but now even Germany is seen as politically unstable.

But the prospect of her departure — she announced this week that she will not run for another term as the German chancellor — has nonetheless created a degree of panic at the core of the European Union.

Ms. Merkel may be becoming more unpopular at home, and her influence over others may be waning. But to those who believed — and worked for — the dream of an ever-closer union, Ms. Merkel was considered fundamentally reliable, decent and committed to Europe and its values. She stood as a bulwark against the strutting populists who now run countries as varied as Italy, Hungary and Poland.

What Europe will do without Ms. Merkel is no small question, especially when nationalism is rising and Europe’s politics seem to be reorganized not along the usual left-right spectrum, but rather around who is for Europe, and who is against it.