Sources say Mr Kumar, returning the Congress Vice President's call, confirmed his support for Gopalkrishna Gandhi, the grandson of Mahatma Gandhi.
The Chief Minister was reportedly unwell when Mr Gandhi had called him after returning from his holiday abroad to complicated opposition dynamics. Sources say the two leaders "discussed various issues".
Mr Kumar didn't attend the meeting of 18 opposition parties in Delhi. He sent Sharad Yadav, his party's other top leader, but clearly his endorsement was important to the opposition bloc.
The Chief Minister and his party had stressed that breaking ranks with the opposition to back the ruling BJP's candidate for President was a one-off and he remained committed to opposition unity.
Gopalkrishna Gandhi was the only name discussed for vice president at yesterday's meeting, a sign that the opposition is taking no chances this time.
When Gopalkrishna Gandhi was earlier considered by opposition parties for the role of President, Mr Kumar had made it clear that he supported the choice. But as the opposition dithered on announcing his name, the BJP picked Ram Nath Kovind, and, according to Mr Kumar's party, he had no choice but to support the then governor of Bihar.
While the Congress has left it to Mr Kumar to decide on whether his deputy should quit, the party has come out in Lalu Yadav's support, backing his claim that he is being subjected to political vendetta. Congress president Sonia Gandhi also spoke to him.
Sources close to the Chief Minister say that he is upset with the Congress' vocal support for Lalu Yadav. His cancelling his Delhi visit yesterday also meant that a planned meeting with Rahul Gandhi was scrapped.
Yesterday, at a four-hour party meeting, Mr Kumar had hinted that he favoured Tejashwi's resignation. So far, Lalu Yadav's party has ruled it out.
Last week, the CBI raided Lalu Yadav's home in Patna alleging that while he was union Railways Minister, he exploited his office to help his family acquire three acres in Patna at dream prices. The land is owned in part by Tejashwi. Lalu Yadav has said that the charges are fabricated.
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