Modi, who had been the deputy chief minister under Kumar, alleged that the senior JD(U) leader talks about principles and propriety in public life in his posts on social networking sites, but does exactly the opposite as he has aligned with the RJD of Lalu Prasad.
"It's this double role of the senior JD(U) leader that the people should guard against while exercising their franchise in the Assembly bypoll on 10 seats," he said in a statement here.
Kumar has made his position "ridiculous" and "untrustworthy" by joining forces with RJD, he claimed.
Under RJD's rule from 1990 to 2005, the state had plunged into "backwardness and darkness" and, ironically, the same party has now aligned with JD(U) which has promised to bring electricity to villages, Modi said.
Besides, the JD(U) leader has accepted RJD's "controversial characters" like Md Shahabuddin, Pappu Yadav and Ritlal Yadav as socialist faces of the alliance, Modi said.
Whatever development has taken place in Bihar occurred during the erstwhile NDA government when BJP was part of it, he claimed adding that it would not be possible without BJP in the future.
The former deputy chief minister also asked Kumar to explain why he did not like Prime Minister Narendra Modi who has promised development of all sections of society.