Merkel's center-left rivals face pressure to name challenger

German Chancellor and chairwoman of the Christian Democratic party, Angela Merkel, addresses the media in Berlin, Germany, Sunday, Nov. 20, 2016. Chancellor Merkel said that she will run for a fourth four-year-term to become one of the longest-serving leaders of post-war Germany. (AP Photo/Markus Schreiber) (The Associated Press)

FILE - In this May 5, 2014 file photo German Sigmar Gabriel, chairman of the German Social Democratic Party, SPD, left, looks to German Martin Schulz, top candidate of the Party of European Socialists, PES, for the European elections and member of the board of the SPD prior to a press conference in Berlin, Germany. Germany's main center-left party faces pressure to decide on a challenger to Angela Merkel in next year's election. (AP Photo/Michael Sohn, file) (The Associated Press)

Germany's main center-left party faces pressure to decide on a challenger to Angela Merkel in next year's election after the chancellor announced she will seek a fourth term.

The Social Democrats lost the chancellery to the conservative Merkel in 2005 and currently serve as her junior coalition partners. They trail in polls, and their hopes appear to depend on a left-wing alliance with the Greens and the Left Party that would have to overcome significant policy divisions.

Party leader Sigmar Gabriel, who is also Merkel's vice chancellor and economy minister, has yet to decide whether to run. European Parliament President Martin Schulz is also considered a strong possible contender.

Germany's top labor union official, Reiner Hoffmann, called for "clarity" from the Social Democrats in comments Monday to the daily Tagesspiegel.