Speaking in a parliamentary debate, Steinbrück made a strong attack on her handling of the euro crisis, saying Merkel had allowed her government to "bully Greece" for months without intervening.
He charged the Chancellor with not having explained the importance of the European project and complained she had not been open to the Germans about the fact that Greece would require more help.
Meanwhile, said Steinbrück, Merkel's government was not doing enough at home to promote growth.
"Rarely has Germany been as isolated as today," Steinbrück told the lower house of parliament, the Bundestag. "Out of a one-sided analysis of the crisis comes a one-sided medicine: save, save, save."
Merkel spoke out again for further united efforts, just hours before the start of a two-day EU summit which kicks off on Thursday evening. She avoided attacking Steinbrück - who was her Finance Minister in the “grand coalition” her conservatives formed with the SPD from 2005 until 2009.
EU heads of state will meet to discuss closer cooperation in economic and finance policy in further attempts to solve the Euro crisis.
The summit will also explore Europe-wide supervision of the banking sector under the European Central Bank (ECB). France and Spain are pushing for this to be put into place rapidly so that finance European bailout funds can be pumped directly into ailing banks, but Germany remains cautious.
Merkel warned against rushing into an ineffective banking supervisory system for banks while also dampening expectations of fast, direct injections of finance to banks by the ECB. She predicted wide-reaching decisions on the Euro would be made in December.
Merkel's further policy suggestions included the creation of a new aid package and the strengthening of the role of the European Commission's top finance official, the Monetary Affairs Commissioner, giving them "real rights to intervention" in national budgets, backing a proposal made by Finance Minister Wolfgang Schäuble.
In many countries, said Merkel, the giving the European Commission the power to veto member states' national budgets would not go down well. "That won't change the fact that we will pursue it," she added.
The Chancellor went on to say that although she saw in Athens a "serious desire for change," Greece had so far failed to push through the austerity reforms requested by the troika, but stated she wants Greece to stay in the Euro zone.
She suggested setting up a fund for supporting reforms in crisis countries as a "new element of solidarity." The money could be used for short-term, project-based costs and could be collected from revenue from a planned tax on financial transactions, she said.
In a recent Forsa polling company study for the Stern news magazine, Steinbrück was given an approval rating of 40 percent.
Yet Merkel retains a clear lead over her rival, scoring far ahead in perceived leadership qualities, and with an overall approval rating of 53 percent. The general election is to be held next September.