Only 36 percent of Germans are currently satisfied with Steinbrück, according to a new Infratest poll published by state broadcaster ARD and Die Welt newspaper. That is a whole 12 percentage points less than at the start of December.
Meanwhile, Merkel's approval ratings rose by another five points to an apparently unassailable 65 percent.
After Merkel, the most popular German politicians are Defence Minister Thomas de Maizière (63 percent), and Finance Minister Wolfgang Schäuble (59 percent), both fellow members of the chancellor's CDU.
The collapse in Steinbrück's ratings have been disastrous since he was named the SPD's candidate at the start of October 2012.
At that time, the former finance minister enjoyed 59 percent approval, and was often polled as being Germany's second or third most popular politician, but negative media coverage about his extra-curricular earnings as a speaker damaged him badly - and have now put him in tenth place of all of Germany's politicians, behind perennially unpopular figures like Foreign Minister Guido Westerwelle.
Since then, his outspoken criticism of politicians' wages and his remark that Merkel gets a "woman's bonus" in the polls have further alienated the population - particularly members of his own party.
The dilemma for Steinbrück is that 82 percent of Germans actually approve of his "I say what I think" approach to interviews - but apparently do not like what he says.
As many as 45 percent of Germans - and 26 percent of SPD voters - think that the SPD should find another candidate.
The parties are facing a crucial bellwether election in the state of Lower Saxony on January 20.