Merkel's Christian Democratic Union (CDU) was at the receiving end of voter anger, suffering defeats in two out of three states in yesterday's elections -- including in traditional stronghold Baden-Wuerttemberg.
The stinging result for the conservative CDU was accompanied by a surge in backing for the right-wing populist Alternative for Germany (AfD), which had sparked outrage by suggesting police may have to shoot at migrants to stop them entering the country.
Today's elections were the biggest since Germany registered a record influx of asylum seekers that reached 1.1 million in 2015, and the vote was largely regarded as a referendum on Merkel's decision to open the doors to people fleeing war.
The mass-circulation Bild newspaper described it as a "day of horror" for Merkel, as calls multiplied for her to change track.
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While acknowledging that some had cast "protest votes", the German leader refused to change her position.
Even though she admitted that Balkan states had done Germany a favour by closing their borders to migrants -- a move that led to a significant drop in new arrivals -- Merkel insisted just days ahead of an EU summit that "we can see from pictures out of Greece that that is not a sustainable solution".
Merkel is increasingly isolated on the European stage as she has refused to impose a cap on arrivals, a decision that is irritating her neighbours who say it encourages migrants to keep coming.
"We need clarity. Everyone must know that it is a false hope to think that Germany would simply wave people in," he told Die Welt daily.