German local vote is test for Merkel's challenger

Two girls run through a field with thousands of dandelions displaying their seed heads in a park in the outskirts of Frankfurt, Germany, Saturday, May 6, 2017. (AP Photo/Michael Probst) (The Associated Press)

A voter casts a ballot in an election room for the Schleswig-Holstein state election in Aukrug-Homfeld, northern Germany, Sunday, May 7, 2017. The Schleswig-Holstein vote is a prelude to an election for the regional parliament in North Rhine-Westphalia, Germany’s largest region, a week later. The two local elections are the last tests at the ballot box before the national contest in September. (Carsten Rehder/dpa via AP) (The Associated Press)

Daniel Guenther, right, top candidate of the Christian Democratic party CDU in the state election in Schleswig-Holstein, and his wife Anke walk to a polling station in Eckernfoerden, northern Germany, Sunday, May 7, 2017. (Marcus Brandt/dpa via AP) (The Associated Press)

Polls are open in northern Germany for a local election that will test the strength of the country's Social Democrats as they try to deny Chancellor Angela Merkel a fourth term.

The Social Democrats are trying to hold onto power in Schleswig-Holstein, where they have governed since 2012. A good showing Sunday in the region of 2.8 million would reinforce their prospects for challenging Merkel in the nationwide election on Sept. 24.

Social Democratic governor Torsten Albig is defending a slim majority for his governing coalition with the Greens and the left-leaning SSW party. Polls point to a tight race with Merkel's Christian Democrats.

The Social Democrats surged in national polls after Martin Schulz, a former European Parliament president, was nominated as Merkel's challenger, but their ratings have since sagged.