The Congress and the BJP say the lawyer-turned-minister must resign immediately or be removed from office by Chief Minister Arvind Kejriwal, who founded the Aam Aadmi Party or AAP a year ago.
In August last year, Mr Bharti and his client were fighting a corruption case in a CBI court in Delhi when the judge said their conduct is "not only highly objectionable, unethical, but also amounts to tampering with evidence."
Mr Bharti has denied the charges. "We made an audio CD to expose those making false claims in the Rs 116 cr case but the court refused to go into the details of the tapes. I don't see this as tampering of evidence. The CBI should catch the real culprits," he told reporters. But he evaded questions on whether, as a defence counsel, it was unethical for him to contact a prosecution witness.
Mr Kejriwal stood by his minister, calling the court observation "completely wrong."
"A sting operation has been called tampering of evidence - it is not tampering of evidence," said the Chief Minister.
"This is a very serious allegation against Somnath Bharti. He misbehaved with his law secretary once. Now there is fresh evidence that he tried to tamper with evidence in an important case," said BJP leader Harsh Vardhan, who was chief ministerial candidate in the December Delhi election.