By-poll to eight vacant Assembly seats due in the state after the death and disqualification of MLAs.
Yadav, who had attributed SP's victory in Gorakhpur and Phulpur to support from BSP and some local parties, spent some at the BSP supremo's residence.
The Congress also claimed the BJP had "lost" its simple majority in the Lok Sabha.
Winning the by-elections for these seats, held by current Chief Minister Yogi Adityanath and Deputy Chief Minister Keshav Prasad Maurya, was a matter of prestige for the BJP.
The Ajit Singh-led RLD, while announcing this here, said it will also support the SP and BSP candidates in the soon-to-be-held Legislative Council and Rajya Sabha elections.