Merkel's critics say she has failed to set any direction and presided over policy drift. Steinbrueck —who was once Merkel's finance minister and says he won't serve under her again — has quipped that if she were in his government, he would give Merkel "the ministry for vagueness."
However, Steinbrueck's center-left has struggled to generate momentum in the face of a healthy economy and a conservative campaign that largely skirted controversy, focusing squarely on Merkel's popularity.
"I have the impression that the (conservatives), Ms. Merkel as well, had an interest in not raising any big issues if possible," Steinbrueck said.
Posters featuring a smiling Merkel declare simply: "Chancellor for Germany."
The opposition's campaign has been marred by problems ranging from criticism of Steinbrueck's high earnings on the lecture circuit to a much-mocked suggestion by his Green Party allies that canteens should introduce a meat-free "veggie day."
And Merkel has attacked plans by the Social Democrats and Greens to increase income tax for top earners, which she says would hurt the economy.
Merkel has brushed aside concerns that a tight finish would weaken her position as Europe's strongest politician, noting that "majorities in Germany are very frequently narrow." She has said the current coalition would continue with "a majority, however big it is."
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