He targeted bickering alliance partner Shiv Sena for "poking its nose" in the BJP's internal affairs, by fielding late MP Chintaman Wanaga's son Srinivas for the byelection.
"The way this party has back-stabbed the BJP by fielding its own candidate...I can say that late Bal Thackeray's soul must be in deep pain and anguish," the Uttar Pradesh chief minister said without taking name of the Sena.
He said the victory of BJP candidate Rajendra Gavit would be a "rich tribute" to the late Sena patriarch.
Virar, one of the six Assembly segments of Palghar Lok Sabha constituency, has a sizable number of north Indian voters apart from Maharashtrians.
"These people invoke Shivaji Maharaj in everything, but in reality they act like Afzal Khan. We won't allow this. Shivaji Maharaj and Balasaheb Thackeray always fought against anti-national elements," he said.
Afzal Khan was a medieval commander who served the Adil Shahi dynasty of Bijapur, and fought against Shivaji. He was killed at a meeting with the Maratha king, and his army was defeated in the Battle of Pratapgad.
The BJP has made the bypoll a prestige issue and has been accusing the Sena of trying to hijack the legacy of late Wanaga.
Without naming Sena chief Uddhav Thackeray, Adityanath accused the saffron party of dividing Hindu votes.
"Balasaheb never believed in back-stabbing. He was a warrior who believed in fighting from the front. He worked for making country stronger. But see what has happened. They (Sena) broke up Wanaga's family and forced him (the late BJP MP's son) to contest the bypoll. This was an internal matter of BJP and we would have protected the self-respect of the Wanaga family," he said.
The actions of this party (Sena) are not in sync with dreams of Chhatrapati Shivaji Maharaj. "People of Maharashtra will teach them a lesson," he said.
"This bypoll won't impact the stability of the government but it will definitely give a message that the country can progress only under Modi's leadership," he said.
The UP CM's remarks came hours after JD(S) leader H D Kumaraswamy was sworn in as Karnataka chief minister at a grand ceremony attended by a galaxy of top leaders and regional satraps in a rare public show of unity perceived as a possible harbinger of a broad-based anti-BJP alliance ahead of the Lok Sabha polls next year.
"I have come here to appeal to you to vote for BJP, whose government under Modi's leadership is taking India on the path of progress," he said.
"The symbol of this progress can be only lotus (BJP's symbol) and nothing else," he added.
"There is an attempt to create an atmosphere against the BJP," he said.
"By God's grace, the Congress has been decimated in the country," he said, adding that the "enemies of development and good governance will also meet the same fate".
The BJP has drafted in a battery of its leaders in Palghar.
Maharashtra Chief Minister Devendra Fadnavis is spearheading the BJP's campaign in urban segments of the constituency.
BJP leaders Manoj Tiwari, Rita Bahuguna Joshi have also addressed rallies.
In his address which he opened in Marathi, Adityanath underlined the "cultural and historical relations" between two states.
"Those who see the two states with different glasses, they seem like anti-India," he said.
Adityanath praised Fadnavis for "making efforts to weed out corruption and 'Gundaraj' in Maharashtra".
He said people of Maharashtra and Uttar Pradesh need not be afraid of anti-social elements.
Recalling his past experience of working with late Wanaga, Adityanath said, "Wanagaji raised issues of the common man in Parliament with full commitment and did several outstanding works. I worked with him and I knew him closely".
Adityanath's stinging attack is likely to further deteriorate already strained ties between the BJP and the Sena, which despite being a ruling constituent seldom lets go any opportunity to target the BJP and the prime minister.
Apart from Srinivas Wanaga and Rajendra Gavit, Congress' Damodar Shingda, Bahujan Vikas Aghadi's Baliram Jadhav, and CPI(M)'s Kiran Gahala are in the fray.
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