For Raja, who was defiantly resisting calls for his resignation at least in public, the call from Karunanidhi came after he landed in Delhi and while he was on his way home, DMK sources said.
The sources said Karunanidhi at one time made up his mind not to ask Raja to resign in the wake of the Opposition's campaign against him over the alleged scandal in 2G spectrum allocation.
But, the sources said, he could not resist the demand after calls from Mukherjee and some other ministers after an informal meeting of the Congress core committee in Parliament on Sunday, where Prime Minister Dr Manmohan Singh and Congress chief Sonia Gandhi held discussions.
The Congress leadership then decided that the DMK patriarch should be approached to cut any further loss in view of the threat by the opposition to stall Parliament proceedings till Raja is removed from the cabinet.
Earlier in the day, Raja had meetings with Karunanidhi when he apparently had no inkling of what was to happen late in the night.
That explains partly his bravado both at the Chennai airport before he left for Delhi and later when he landed in the capital when he told journalists that he was neither going to meet the PM nor was he going to resign.
But shortly later, apparently after a call from Karunanidhi, Raja called up his personal staff to type his resignation letter to be given to the PM. He then broke the news of his resignation and went to the 7, Race Course Road official residence of Dr Singh to do the needful.
