"Nitish came to power by saying he was fighting against 'jungle raj' of Lalu Prasad. Now he has gone and sat in the lap of same Lalu without any hesitation...There is no shame," he said addressing the BJP National Council here.
"In politics, principles matter more than numbers.. We need to expose this immoral alliance," Shah told the gathering of leaders and cadres.
Arch rivals JD(U) and RJD came together after their drubbing in the Lok Sabha polls and have decided to contest the upcoming by-elections in alliance along with Congress.
Shah exhorted party workers to work hard and ensure that BJP wins majority on its own in the state where Assembly elections are due next year.
Nitish Kumar's party JD(U) had broken its long time alliance with BJP last year after the latter announced that Narendra Modi will be its chief campaigner for 2014 Lok Sabha elections.
After break-up, JD(U) suffered a serious setback in the Lok Sabha elections as majority of the seats in the state were corned by BJP and its alliance partner LJP.
As part of the seat-sharing for 10 by-poll seats, JD(U) and RJD are fighting on four seats each while Congress has fielded candidates on two seats.
In the NDA camp, BJP is contesting on nine seats while ally LJP has fielded its candidate on Parbatta seat.
Of the 10 seats going to by-polls following election of some to Lok Sabha and resignations of MLAs in the other seats, six were with BJP, three with RJD and JD(U) one.