Lalu's party is the biggest faction of the Bihar government, but though it won more seats than Mr Kumar's Janata Dal (United), it honoured a pre-election commitment for him to lead the state. The Congress is their junior partner.
Yesterday, Mr Kumar, 66, gave a leg up to his allies' complaints of him crossing the line to side with the BJP, a party with whom he allied for 17 years before a bitter break-up in 2013.
Now, sources in the Congress and Lalu believe, another split is imminent. They say Mr Kumar, aware of the huge popularity of Prime Minister Narendra Modi, is pedaling back to his side before the next general election in 2019.
Mr Kumar's party flatly denies political shape-shifting and says it's backing of the BJP's candidate for President is based entirely on the merit of Ram Nath Kovind, who was Governor of Bihar. Mr Kovind, 71, is a Dalit, a caste that forms the main support base for Mr Kumar - he expanded the category about a decade ago to include more backward Dalit castes and introduced more welfare schemes for them.
"Our Maha-gathbandhan will not be affected," insisted Sharad Yadav, a top leader of Mr Kumar's party, referring to the Grand Alliance that was meant to group anti-BJP parties against PM Modi, with the election for President of India as the genesis. But by exiting the opposition bloc, which had 17 parties, Mr Kumar has pulled on a thread that's unraveling much more. The remaining opposition parties are meeting in Delhi today to decide whether to put up a candidate of their own for President, which means an election would be held on July 17.
At one time, Lalu was the undisputed powerhouse of Bihar. In the 90s, his leadership of the state came to be known for corruption and jungle raj or lawlessness. When he was forced to resign as Chief Minister because of corruption charges, he installed wife Rabri Devi, innocent of any political experience, as his proxy. In 2005, it was Mr Kumar who ended the 15-year run that Lalu had enjoyed - he took over as Chief Minister with the BJP as his partner.
In 2013, Mr Kumar aborted their lengthy collaboration claiming that the BJP was wrong to pick Mr Modi, who was then Gujarat Chief Minister, as its candidate for Prime Minister. Colossal misstep. Mr Kumar's party won just two of Bihar's parliamentary seats. The BJP, riding the indisputable Modi Wave, won more than 30.
Chastened, Mr Kumar binned his hostility with Lalu in 2015 and they decided to contest the Bihar state election with the Congress in tow. The combination of their support groups proved to be the wall that blocked the PM from leading the BJP to taking over Bihar.
Sources in Lalu's party say he will not accept any proposal for his sons to quit as ministers till they are cleared by a court. Once they are chargesheeted - a court is informed by investigators of their alleged crimes with accompanying early evidence - the Chief Minister is likely to want their resignations.
Those that speak of the gap between Lalu and the Chief Minister point also to the fact that over 100 bureaucrats remain in posts much after their three-year term. Lalu is allegedly determined to give some key offices to candidates of his choice. Mr Kumar has reportedly refused. The stand-off means that new appointments and transfer postings are not being made.
Mr Kumar is also reportedly annoyed that Lalu's sons stall projects that he is personally invested in. For example, the Ganga Driveway, a highway that would be constructed over the river, has been delayed because Tejashwi Yadav has changed the alignment and sought fresh bids for contracts.
Sand mining, one of the most lucrative businesses in the state, was investigated recently on the orders of the Patna High Court and it was legislators from Lalu's party that were found to be involved with suspicious deals.
But few issues ripped off the bandage for Lalu and Mr Kumar like PM Modi's surprise decision in November to ban high-denomination notes. While every opposition leader including Lalu publicly flogged the decision for causing hardship to the rural poor and forecast it as the PM's downfall, the Bihar Chief Minister was unwavering in his endorsement, describing it as a significant step in fighting corruption and tax evasion.
His comments forced Lalu and the Congress to seek clarity from him on whether he was contemplating a reconciliation with the BJP. He assured them he was not. But his consent to the BJP's candidate for President ties a bow on his allies' concerns. For Lalu, in particular, who wants to remain in power while his children are investigated, the alliance has never seemed this wobbly.