With this, Yadav also put to rest speculations that the Samajwadi Party may once again join the JD(U)-RJD-Congress grand alliance to contest the Bihar Assembly elections.
The SP had only last week decided to quit the grand alliance in Bihar over seat-sharing issues.
Questioning Kumar's "secular credentials", Yadav said that the JD(U) leader "easily" changed friends and allies, having enjoyed the support of BJP till recently.
"Look at what is happening in Bihar. SP has always consistently opposed BJP...Then who is secular? People who ran government with BJP's support for 12 years? Now they are secular all of a sudden," Yadav said while addressing a conclave of his party's women's wing -- the SP Mahila Sabha -- here.
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Bihar SP president Ramchandra Singh Yadav had yesterday said the party will contest all 243 seats alone in the upcoming polls.
"SP chief Mulayam Singh Yadav, listening to sentiments of workers, went by the decision of the party's Parliamentary Board to end ties with secular alliance of JD(U), RJD and Congress, and contest alone in Bihar," Yadav told reporters after a meeting of party districts chief here.