BERLIN (AP) — German Chancellor Angela Merkel and her center-left rival Martin Schulz are preparing for their only televised election debate Sunday before the country's Sept. 24 vote.

Merkel, who heads the center-right Christian Democrats, has been stepping up her campaign in recent weeks as she seeks a fourth term leading Europe's biggest economy.

Schulz was the president of the European Parliament until his Social Democratic Party unexpectedly nominated him in January to challenge Merkel.

A poll published Friday by public broadcaster ZDF puts the Social Democrats at 22 percent compared with 39 percent for Merkel's Union bloc. The survey of 1,309 German voters had a margin of error of up to 3 percentage points.

The parties are currently in a coalition at the national level, but have expressed a desire for change.