Sachin Ahir was a cabinet minister in the previous Congress-NCP government in Maharashtra

A top leader of the Nationalist Congress Party, Sachin Ahir, joined the Shiv Sena today, in a big jolt to the opposition party just months before assembly election in Maharashtra.

Sachin Ahir, who was NCP's Mumbai chief, joined the Sena in the presence of its leader Uddhav Thackeray and his son Aaditya Thackeray.

"In politics, tough decisions have to be taken," the 47-year-old former lawmaker told reporters.

Acknowledging that the transition would be "difficult" given the ideological differences between the two parties, Sachin Ahir told NDTV, "I'm a good political student. So I'm going to start from first now, junior KG, senior KG."

Mr Ahir's move will weaken his former party in Worli in Mumbai, where he was a strong rival for the Shiv Sena despite its formidable cadre base.

But the former lawmaker from Worli made it clear that the candidate for the constituency in the upcoming assembly elections "will be of the Shiv Sena and the winning candidate will be of Shiv Sena".

He may, however, face the opposition of Sena workers who have seen and fought him as a rival for decades.

"We won't use him only in Mumbai, he will campaign for the party across Maharashtra," said Aditya Thackeray, to questions of a potential divide within the ranks over Mr Ahir.

To a question on whether he approached the Shiv Sena or it was the other way round, Sachin Ahir said, "Aditya and I always used to interact, we used to criticise each other but we used to talk also on some common agenda but we never thought of getting together. Then we met, he said, I said 'this is the common agenda, why not work together'".

Sources have told NDTV that other senior NCP leaders are also in talks to switch to the Sena. Among them is Chhagan Bhujbal, a former Sena leader who was Deputy Chief Minister in the Congress-NCP government. Mr Bhujbal, who was jailed on money laundering charges, is considering and negotiating a return to the Sena but his former party is reportedly not convinced about taking him back.

Mr Bhujbal, who was in the Sena for 25 years, joined Sharad Pawar when he formed the NCP. He was part of the decision to arrest Sena patriarch Balasaheb Thackeray, which remains a major thorn for hardcore 'Sainiks' who oppose his return. Several top leaders have reportedly advised Uddhav Thackeray against taking Mr Bhujbal back.