"It is a conjuring trick to promise tax reductions worth between €20 billion and €100 billion in the middle of an economic crisis," Steinmeier told a readers' forum in Sunday's Bild am Sonntag newspaper. "There is no way we would be able to afford better schools and universities."
Steinmeier went on to accuse Merkel's Christian Democratic Union (CDU) of "window-dressing" politics to hide the lack of a consistent agenda. "It is difficult to see what the CDU actually stands for," he said.
Steinmeier, currently serving as foreign minister in Merkel's coalition government, said that dismantling the university fee system would be a priority if the SPD won the general election on September 27.
"I would not be sitting here today if there had been university fees in the 70s," he said. "My parents, like many working class families, would not have been able to afford my education. That's why we need to get rid of all fees on all levels from kindergarten to university as quickly as possible."
But if he became chancellor, Steinmeier would not be able to make sweeping reforms like this since education financing in Germany is administered by individual states. At the moment, university fees only exist in states run by the CDU, and Steinmeier hopes that a SPD-led government would persuade those states to introduce free education.
Steinmeier also used the forum to defend his party chairman Franz Müntefering, who attracted criticism recently for his blunt statement that Merkel "did not care about the nation's unemployed."
"If the chancellor-candidate of the CDU can call my target of full employment by 2020 "dishonest," then Müntefering is allowed to retort with his own rough judgments," Steinmeier said.