After Schröder had to call for new elections in 2005, it wouldn't go down well in the public if the SPD is seen as deliberately giving up the government AGAIN. It would have to be about a very popular and divisive issue where the SPD can get more support for its side AND they'd have to put the blame on Merkel somehow. Not exactly promising either.

"If you know your enemies and know yourself, you will not be imperiled in a hundred battles." Sun Tzu