The leader of Germany’s Social Democrats, the junior partner in Chancellor Angela Merkel’s governing coalition, announced her resignation on Sunday, raising new questions about the government’s future, a week after the parties lost support in the elections for the European Parliament.

The party leader, Andrea Nahles, said in a statement sent to Social Democratic members on Sunday, “The discussions within the parliamentary faction and feedback from within the party have shown me that I no longer have the necessary support to carry out my duties.”

She said she would formally hand in her resignation on Monday.

Political observers noted that Ms. Nahles’s departure could strengthen the left-wing of the Social Democrats, spelling doom for the party’s willingness to remain in the unloved coalition government.

“The situation is dramatic,” said Thorsten Faas, a professor of political science at Berlin’s Free University. “The opponents of the grand coalition within the Social Democrats clearly have the momentum right now.”