Germany's right-wing populist Alternative for Germany (AfD) party ramped up attacks against immigration and Islam.
The Muslim religion "does not belong in Germany", said a leading AfD candidate Alexander Gauland, who argued its "political doctrine is not compatible with a free country".
"Islamist rhetoric and violence and terror have roots in the Quran and in the teachings of Islam," he told reporters on Monday.
Among other proposals, AfD wants a ban on minarets and public calls to prayer from mosques, a ban on headscarves for teachers and students, and for imams to lead prayers only in German.
Merkel's Christian Democrat alliance CDU/CSU slipped two points to 36 percent, close to the all-time low of 35 percent when the Social Democratic Party of Germany (SPD) led by Gerhard Schroeder defeated it in 1998.
Merkel's conservative alliance still commands a huge lead over the SPD of her top rival Martin Schulz, which slipped to 23 percent.