BERLIN (AP) — Germany's main center-left party faced pressure Monday 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 partners in a "grand coalition" of Germany's biggest parties.

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.

Whoever takes on the task faces an uphill struggle to dislodge Merkel. The Social Democrats trail Merkel's Union bloc by 10 percentage points or more 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.

Gabriel is tactically skilled but far less personally popular than Merkel. Schulz has raised the European Parliament's profile and helped secure one of the party's best results in recent years in 2014 European elections, but is less well-known at home.

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

It is "time for party chairman Sigmar Gabriel to say whether he is running as a candidate for chancellor," he added.

Ralf Stegner, a deputy party leader, said that Gabriel would propose a candidate in due time, but didn't say when.

"We will talk calmly about when and how we do it — it won't take forever, but we won't let ourselves be pushed," Stegner told ZDF television. "We will make the right decision, and one with which we have good chances of beating the Union."

A three-way left-wing alliance, even it wins a majority, is a difficult proposition because the Left Party grew out of opposition to economic reforms conducted by a Social Democrat-led government and opposes military deployments abroad.

While they have worked together in state governments, the three parties are only gradually moving closer at the national level. And some Greens would prefer to link up with Merkel's conservatives.

Stegner conceded that he couldn't rule out a repeat of the current coalition — "but we don't want a 'grand coalition.' Anyone who says they do in an election campaign can hand out sleeping pills."