Having politicians set minimum wages is "absolutely wrong it can't work out well," Labor Minister Ursula von der Leyen, a member of Merkel's Christian Democrats, told ZDF television. "We say that the people who are experts unions and employers should do it."
Steinbrueck's campaign spokesman for labor issues, Klaus Wiesehuegel, accused the government of delaying a decision on approving the latest deal only to clear it days before the election.
"That's not how credibility looks," he said.
Merkel is heavily favored to win a third term in Sunday's parliamentary election, but the prospects of continuing her partnership with the Free Democrats are uncertain. The smaller party strongly rejects having a national minimum wage.
Polls this week have shown the coalition of Merkel's conservatives and the struggling Free Democrats level with or marginally behind the combined main opposition Steinbrueck's Social Democrats, their Green allies and the hardline Left Party.
Merkel's conservatives look set to emerge as the biggest single party.
The Social Democrats and the Greens together are short of a majority and say they won't form a coalition with the Left Party, which opposes German military deployments abroad as well as eurozone reform and bailout policies. The Left Party also seeks more radical redistribution of wealth and demands a 10-euro ($13.36) minimum wage.
That leaves as likely a Merkel-led coalition of her conservatives with the center-left Social Democrats. That would be a return to the "grand coalition" that Merkel led from 2005 to 2009.