The remarks was made by senior JD(U) leader Shakuni Chaudhary whose son Samrat Chaudhary is a minister in the Jitan Ram Manjhi government.
Coming up strongly against the JD(U)-RJD tie-up, Chaudhary told reporters that Lalu Prasad and Nitish Kumar have become "arrogant" and undermining him.
"Both of them who came to me for help in the past are today drown in arrogance and have forgotten me," he said.
Referring to speech of the RJD president at Bhagalpur yesterday that Nitish Kumar was "kidnapped" by BJP for 17 years and after coming out of that he (Kumar) fell on his feet seeking alliance with RJD. As an elder brother no option was left for me but to accept his (Kumar) plea.
Chaudhary said "in the wake of such utterances from Prasad, if Kumar has any self-respect he should quit politics or take steps to protect his honour."
He said this was not an isolated incident as Prasad has been making light of Kumar in all election rallies.
Asked about prospect of the secular alliance in the bypoll, Chaudhary painted a grim picture and said "history is testimony to the fact that arrogant people have always doomed."
Chaudhary had switched over to JD(U) after his son led a group of RJD MLAs to rebel early this year.
Chaudhary's blunt comments on Nitish Kumar has not been taken kindly by the party.
JD(U) president Sharad Yadav, who is hear to campaign in the bypoll, told reporters that he would talk to Chaudhary about his comments.
JD(U) spokesman Sanjay Singh told reporters that Chaudhary has grown old and was in habit of speaking anything.
"The train of alliance has begun journey...those who do not want to ride the train shall get down on the platform as this journey will stop only on reaching Delhi," Singh, a close confidante of Nitish Kumar, said.