"Singh knew what went wrong in the 2G spectrum allotment, but did not stop it," Yadav said at a state-level conference of JD (U) here.
"He (Prime Minister) is the guardian of the country and should have stopped this loot," the NDA convener added.
"Singh is a good person but he should have displayed his righteousness and nipped the 2G scam in the bud," Yadav said.
"What is the use of a good cow if she doesn`t give milk?" he quipped.
He said the democracy had been weakened due to the recent spate of scams.
"The honesty is missing... even if Jaiprakash Narayan or Ram Manohar Lohia were to come back and contest an election, they would lose," Yadav said. "Votes are now being bought."
However, he asked the audience not to lose heart, saying that 27 people including some corporates bosses were cooling heels in jail on corruption charges.
"The opposition is united, like a solid rock and the judiciary is very actively dealing with corruption," he said, adding that "there is an awakening among the people against corruption."