RLD candidate Tabassum Hasan defeated her nearest rival BJP's Mriganka Singh by over 55,000 votes.
After Kairana results, RLD leader Jayant Chaudhary said that the united opposition had succeeded in halting the "chariot of hate" of the BJP in Uttar Pradesh. He also hoped the combined opposition would keep its momentum to defeat the BJP in the Lok Sabha elections next year.
By consolidating the anti-BJP vote, the opposition repeated its success in Gorakhpur and Phulpur bye-elections earlier this year when the ruling party suffered humiliating defeats. Bye-elections were held for Gorakhpur and Phulpur in March as the seats fell vacant when UP Chief Minister Yogi Adityanath and his deputy Keshav Prasad Maurya resigned after entering the UP legislative council.
Kairana was crucial for the BJP because the by-election was necessitated by the death of its towering leader Hukum Singh, whose daughter Mriganka was made the party candidate. The ruling Bharatiya Janata Party has campaigned hard for Kairana.
After Phulpur-Gorakhpur, Kairana was seen as a test case for the sustainability of the alliance as internal contradictions are far more complex in the western part of the state. But the choice of candidate demonstrated exceptional politico-cultural assimilation and the resolve of the leaders to stay together.
The candidate - Tabassum Hassan, who won the Kairana Lok Sabha seat in 2009 from the BSP - is now in the SP but contested on an RLD ticket. She joined the RLD only on May 7 after Akhilesh and Jayant cleared the plan. At a public meeting, following which she filed nomination papers on Wednesday, supporters of the SP, BSP, RLD and the Congress were also present.
Tabassum is the wife of former Samajwadi MP Munnawar Hasan. Their son Nahid Hasan is an SP legislator. It is unique in that sense: a candidate so entwined with the SP and the BSP is contesting on the RLD symbol.
After the 2013 riots in Muzaffarnagar, the social fabric of the region was in tatters. The candidacy was also an attempt to bring the dominant Jats together with Muslims and Dalits, a traditional rainbow coalition of social groups the region was known for.
The opposition's bonhomie was on display when HD Kumaraswamy took oath as the chief minister on May 23. The show of strength by a galaxy of national leaders and regional satraps was seen as a strong signal to the BJP about the possibility of formation of a broad-based front to take on the BJP-led NDA in the Lok Sabha polls next year.
The BJP had swept 73 out of the 80 Lok Sabha seats in Uttar Pradesh in 2014 general elections and won an overwhelming 312 seats out of a total 423 in the state assembly elections last year.
The Kairana election is the fourth Lok Sabha bye-election in Uttar Pradesh since 2014. Apart from Gorakhpur and Phulpur, a bye-election was also held in Mainpuri in 2014. Samajwadi Party patriarch Mulayam Singh Yadav had resigned from Mainpuri after deciding to represent Azamgarh, the second constituency from which he won in 2014.
SP's Tej Pratap Singh Yadav, who is related to him, defeated the BJP there in that bye-election, retaining the seat for his party.
(With inputs from The Telegraph Calcutta and PTI)