BERLIN — Chancellor Angela Merkel has been a seemingly invincible figure in German politics. In office 13 years, she has been Europe’s most powerful leader, a presence so synonymous with stability Germans call her Mutti, or Mother. So it was a familiar sight on Monday to see her live on television, until she asked Germans to do something far less familiar, and “get ready for the time after me.”

The chancellor said she would step down as leader of her conservative party in December and would not seek re-election in 2021. That means Ms. Merkel may remain on the political scene for months to come. But few observers believe she could hang on until the end of her term, speculating that new elections could be held as early as next year.

The chancellor’s decision now makes clear that neither she nor her country are immune to the forces that have reordered politics across the Continent — the cratering of the political center; the rise of populist forces; the blowback from the migration crisis; and a redrawing of the political fault lines away from the historical left-right divide toward a battle between liberal pro-European values and their nationalist polar opposite.

Speculation had grown for months about Ms. Merkel’s eventual exit from the political stage, so the announcement was no surprise, but it still came as a shock. It underscored the new fragility of German politics and the great uncertainty for a Europe without Ms. Merkel at the helm.