Nitish Kumar deserves a special mention here as he has played an important role in Advani's scheme of things.
Political commentators have always believed that it was Advani's covert support that encouraged Nitish Kumar to take an overt anti-Modi stand. In other words, Advani used Nitish as a bulwark against Modi.
What is interesting is the fact that Advani did not resign from the position of NDA chairman and it was a well calculated move. Advani knew that Modi will never be able to get a majority on his own and parties like JD(U) will play a crucial role in the post-election scenario. To make a last-ditch effort for realizing his lifelong ambition of becoming PM, it would be an opportunity for him to leverage his good equation with BJP's allies.
Apart from Nitish Kumar, all other allies of BJP have made it clear that they have no problem with Modi. AIADMK supremo Jayalalitha, who is not a part of NDA, has already congratulated Modi and is widely expected to support Modi in a post-poll scenario. Advani is aware of all these realities, but still he decided to precipitate matters by orchestrating his resignation.