Modi's Bharatiya Janata Party (BJP) led in 113 of 288 seats in Maharashtra, NDTV news said. That more than doubled its seat count in the western state that is home to Mumbai, the financial capital, but fell short of an outright majority.
The BJP was ahead in 50 of 90 seats in Haryana, which borders the capital, New Delhi, enough to rule alone. The party's showing was just down from the May general election, when it won India's first parliamentary majority in 30 years.
The 64-year-old Modi, a gifted stump orator, hit the campaign trail hard and will be able to reap capital from the victories, although the BJP did not achieve its ambition to rule Maharashtra alone.
"There is a Modi wave," said Information and Broadcasting Minister Prakash Javadekar. "People want a leader who can deliver, people want a leader who is honest."
Both states, which voted on Wednesday, were bastions of the Congress party that has long dominated Indian politics. As in the general election, Congress was decimated, and risks sliding into oblivion under mother-son duo Sonia and Rahul Gandhi.
"Today is likely to be both a BJP win and a funeral of a 150-year-old party," said investment manager and columnist Surjit Bhalla, referring to the party founded in 1885 and led by the Nehru-Gandhi dynasty for four generations.
Modi had already seized on exit polls giving the BJP a clear lead to shake up his economic team, replacing the top civil servant at the finance ministry and hiring U.S.-based economist Arvind Subramanian as his chief economic adviser.
His government on Saturday scrapped diesel price controls and raised the cost of natural gas, giving market forces greater sway as it seeks to attract energy investment, boost competition and cut subsidy costs.
Senior government officials say Modi may soon beef up the cabinet he formed at the end of May, to ease pressure on heavyweights like Arun Jaitley, who holds both the finance and defence portfolios.
Jaitley, recovering from a recent stomach operation, has his work cut out to draft an annual budget by February to encourage business investment and revive Asia's third largest economy from a prolonged slowdown.
State elections determine seat shares in the upper house of parliament, where the BJP and its allies lack a majority. The BJP hopes for gains in a clutch of state polls until 2017 to get the majority it needs to pass some legislation.
If final results show the BJP short of a majority in Maharashtra, it would probably team up with the regional Shiv Sena party, which is still part of its national coalition. The two parties fell out during the state campaign.
According to a partial official count, the BJP had won or was leading in 113 seats in Maharashtra, where it polled 26.6 percent of votes.
In Haryana, it had won or was ahead in 50 seats, with a vote share of 33.6 percent, the Election Commission of India said.
Final results were due later on Sunday.