They were the monthly survey's weakest ever figures for the coalition parties, ARD said.
The poll also relegated the SPD to fourth place, behind the pro-immigration Greens, up 3 percentage points on 17 percent, and the anti-immigration Alternative for Germany (AfD) party, unchanged on 16 percent.
The results chime with expectations that the CSU, the Bavarian sister party to Merkel's CDU, is heading for its worst showing in a state election in over 60 years on Sunday, a setback that risks widening divisions within the coalition.
Since it took office in March it has twice come close to collapsing, due to arguments over immigration and a scandal over accusations of far-right sympathies against Germany's former domestic spymaster.
The coalition is also embroiled in a row over how to phase out polluting diesel cars.
Pollster Infratest dimap conducted the survey of 1,508 people from Monday to Wednesday, ARD said. (Reporting by Michael Nienaber, editing by John Stonestreet)
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