The Bharatiya Janata Party left its ally Janata Dal-United far behind in the seat tally in the Bihar assembly elections, emerging as the senior partner in the alliance for the first time in about two decades.
The development can change the power dynamics within the ruling alliance, even if it manages to retain power in the state and Kumar continues to be chief minister.
Political watchers believe that the BJP after playing the second fiddle in the alliance since it came to power in 2005 will now be more assertive in pushing its agenda.
All eyes will be on Kumar, who has never shown any particular liking for its ally's Hindutva push, as to how he responds to the bitterly changed realities for his party which had won 71 seats in 2015 as part of an anti-BJP alliance before he switched sides in 2017.
He had broken his 17-year old alliance with the BJP in 2013 over the elevation of Narendra Modi as its prime ministerial candidate by the saffron party.
As Modi led the National Democratic Alliance to a huge victory in the 2014 Lok Sabha polls while the Bihar chief minister's differences grew with the Rashtriya Janata Dal, it prompted Kumar to join hands with the BJP.
In the 2010 assembly polls, when the JD-U and the BJP were together, they had won 115 and 91 seats respectively.
The corresponding figures in the October 2005 polls were 88 and 55.
In February 2005 polls, which had thrown up a hung assembly, the JD-U had won 55 and the BJP 37.
The BJP had won more seats than Kumar-headed Samata Party in the 2000 polls by pocketing 67 seats while its ally got 34.
Jharkhand was then part of Bihar. The Samata Party had later merged with the JD-U.
The JD-U has been hit hard in these polls by Chirag Paswan-led Lok Janshakti Party, which was instrumental in its defeat in at least 30 seats.
The LJP was, however, managed to win only one seat.