Updated

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.