The Indian prime minister Narendra Modi’s rightwing party claimed victory in elections in two key states on Sunday, tightening its grip on power after winning national elections in May.

The Bharatiya Janata party (BJP) was leading in Maharashtra, of which the financial hub Mumbai is the capital, beating its centre-left rival Congress party which has ruled the western state for 15 years.

“BJP will definitely form the government in Maharashtra,” the BJP’s state president, Devendra Fadnavis, told reporters in Mumbai as the vote count continued.

The BJP also won in northern Haryana, which borders New Delhi, after 10 years of Congress rule of the state. The state’s outgoing chief minister, BS Hooda, said: “Like the Congress earlier got the mandate, now the BJP got the mandate and will form the government.”

Modi campaigned doggedly for the elections held last week, and the victories are likely to encourage him to push ahead with promised reforms. He won the general election on pledges to revive the ailing economy and clean up endemic corruption, but many of the reforms have yet to be introduced.

On the eve of the state results, Modi’s government lifted controls on diesel prices, aiming to give market forces greater influence on the economy, attract investment and cut subsidies.

The victories will strengthen the party’s power in the national parliament’s upper house, crucial for the passing of contentious laws. The BJP currently lacks a majority in that chamber, whose composition is based on seats won in regional assemblies.

The Delhi-based political analyst Samir Saran said the victories “allow greater space to Modi to accelerate his reforms agenda. In many ways the results signify the continuing rejection of the brand of politics on offer from the Congress and its allies at the centre and in the states. It also is confirmation of Narendra Modi as the leader with momentum.”

The BJP is expected to fall short of an outright majority in Maharashtra and could need a partner to form government. It is expected to mend ties with its ally of 25 years in Maharashtra, the far-right Shiv Sena, after deciding to campaign alone following an acrimonious split.

“I hope that the old alliance is restored once again in a crucial state like Mahrashtra,” the BJP veteran leader LK Advani said.

The BJP had won or was leading in 120 seats in Maharashtra, in tallies on the election commission’s website, while Congress was trailing with 43 in the 288-seat state assembly. Shiv Sena was leading in 59 seats.

In Haryana, the BJP was ahead in 49 of the 90 seats up for grabs, while Congress was leading in 15.