The party said Yadav has "voluntarily" given up JD(U) membership by attending the rally organised by Lalu Prasad's RJD.
"As per provisions of the 10th schedule of the Constitution dealing with anti-defection, if somebody attends a rally of another party against the party line it is contrued that he/she has voluntarily given up the membership of the parent party," JD(U) Principal General Secretary K C Tyagi said.
JD(U) sources made it clear that the party will not expel Yadav, a founding member of the party and its longest serving president, as it would allow him to retain his Rajya Sabha membership.
He was elected to the Rajya Sabha last year and his tenure ends in 2022.
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The Rajya Sabha membership of former MPs like Capt. Jai Narayan Nishad and Upendra Kushwaha had ended in similar circumstances in the past, he said, adding the party would soon approach Rajya Sabha chairman Venkaiah Naidu for Yadav's disqualification.
Yadav was cosying up to RJD ever since JD(U) chief and Bihar Chief Minister Nitish Kumar walked out of the Grand Alliance in Bihar and formed government with BJP.
The JD(U) had replaced Yadav with a Nitish Kumar confidante R C P Singh as its leader in the Rajya Sabha a few days ago.
Earlier in the day, Yadav was warmly received by Lalu at the rally in Patna. Lalu embraced Yadav while his son Tejaswi Yadav addressed him as "guardian".