In elections in 2002 and 2005 that healthy lead fell away sharply.
If the popularity of the CDU is weakened, Merkel may be pushed into forming a coalition with the SPD, jeopardising her plans to push through tax cuts and legislation to extend the life of the country's nuclear power plants.
Her preferred option is to form a coalition government with the business-friendly Free Democrats (FDP).
An opinion survey carried out over the weekend indicated that 87 per cent of Germans expect Merkel to win a second term as chancellor.
In a third regional vote in the eastern state of Saxony, Merkel's party is expected to retain power.