The far-right Alternative for Germany (AfD) came in third with eight percent of votes.
The Christian Democrats and the Social Democrats, who formed a grand coalition government over three years ago, have been running neck-and-neck in polls since February.
The SPD briefly pulled ahead by one percent early last month, a historic success that was attributed to the popularity of its new leader, Martin Schulz, as a new face on the German political arena.
The parliamentary elections in Germany will take place on September 24, 2017.