Karnataka Chief Minister Siddaramaiah said unity among the non-BJP parties has played a key role.
On a bad day for Uttar Pradesh Chief Minister Yogi Adityanath, his Karnataka counterpart and sparring partner Siddaramaiah poked him on Twitter, urging him to "spend less time lecturing" his state.
Hours after the UP bypoll results, in which the BJP's seats of Gorakhpur - Yogi Adityanath's five-time parliamentary constituency - and Phulpur went to the Samajwadi Party,
Mr Siddaramaiah took a swipe at the saffron-robed politician in tweets.
"BJP has suffered a humiliating loss in the Lok Sabha seats held by the CM & Dy CM of UP. Congratulations to SP & BSP for this historic victory. Unity among the non-BJP parties has played a key role," he tweeted.
"Perhaps Yogi Adityanath should spend less time lecturing Karnataka on development," he sneered.
His Congress party followed suit with another stinging post.
The back-and-forth between the two chief ministers began when Yogi Adityanath, the BJP's Hindutva mascot and key campaigner, visited Karnataka in January to address a rally for the state elections ahead.
Mr Siddaramaiah and Yogi Adityanath hit out at each other and exchanged barbs over the state of development and governance in Karnataka and Uttar Pradesh.
"There is a lot you can learn from us Sir. When you are here please visit a Indira Canteen and a ration shop. It will help you address the starvation deaths sometimes reported from your state," Karnataka Chief Minister had tweeted, "welcoming" Yogi Adityanath. The UP Chief Minister retorted: "I heard number of farmers committing suicide in Karnataka was highest in your regime, not to mention the numerous deaths and transfer of honest officers."