The basic contours of Germany’s election campaign are that absolutely everyone expects the Christian Democrats to secure a plurality of seats in the Bundestag. What’s more, everyone knows that the CDU’s first choice is to form a coalition with the libertarianish Free Democrats. So if CDU+FDU holds a majority, that’s what you’ll get. If they don’t hold a majority, then the CDU will form a grand coalition with the Social Democrats. Since there’s a grand coalition in office right now, somewhat paradoxically if you want the status quo to continue what you need to do is vote for the main rival of the currently ruling political party. Alternatively, if you want to do what the Chancellor wants you need to vote for the largest official opposition party in parliament.
What’s a bit odd is that as best I can tell the SPD feels they just can’t make this argument explicitly. Instead of running a pro-status quo campaign against the FDP’s unpopular free market economics, they’re basically trying to maintain a pretense of running a change campaign against Angela Merkel and the CDU. I suppose I understand why they feel they can’t pivot in this direction, but it seems like a bit of a tactical error.