BERLIN (AP) — German Chancellor Angela Merkel is embarking on a complicated quest to form a new government and find answers to the rise of a nationalist, anti-migrant party.

Sunday’s election in Europe’s biggest economy left Merkel’s conservative Union bloc weakened after a campaign that focused squarely on Germany’s leader of the past 12 years. However, no other party can lead a new government and Merkel herself lacks any obvious internal challenger.

Merkel’s partners since 2013 in a “grand coalition” of Germany’s traditionally dominant parties, the center-left Social Democrats, vowed to go into opposition after a heavy defeat.

Caucus leader Thomas Oppermann doubled down on that pledge Monday, saying that “we will not conduct coalition talks, because voters have decided that the Social Democrats’ place is in opposition.”