Two regional parties that were former bitter rivals announced an unlikely alliance Saturday to fight Indian Prime Minister Narendra Modi's ruling Hindu nationalist party in a looming general election.

The Samajwadi Party (SP) and the Bahujan Samaj Party (BSP) — key players in the northern state of Uttar Pradesh — said they would set aside their differences to jointly fight Modi in the bedrock state.

The two central-left parties have widespread support among lower castes and poor voters across the state — India's most populous, with 220 million people.

Read: Indian PM Modi's BJP loses key state elections

Uttar Pradesh is a bellwether of national politics, accounting for 80 of the 552 members of parliament in New Delhi. An election is expected to be held in April and May and one recent poll indicated Modi's Bharatiya Janata Party (BJP) may fall short of a majority.

Akhilesh Yadav, the Samajwadi Party head, said the alliance would counter the “divisive politics” of the BJP, which won a landslide victory in 2014.

“The BJP is dividing the country, it is stoking fear and hatred among communities,” Yadav told a news conference sat alongside Mayawati, a popular low-caste leader who heads the BSP.

The parties, which will contest 38 seats each out of the state's 80, left the main opposition Congress party out of the alliance.

The two have been fierce rivals in recent years. They teamed up in 1993 and formed the Uttar Pradesh state government but relations soured after Mayawati said she was assaulted by Samajwadi Party activists in 1995.

Modi's BJP suffered a rare reverse when it lost three key state elections to Congress last month, amid discontent over unemployment and economic inequality.

A BJP spokesman played down the importance of the alliance.

“We are confident. Even if all the parties come together, we will still win,” Sudhanshu Trivedi told reporters in Delhi where the BJP is holding a key convention.

Modi rallied 10,000 party workers at the convention, dismissing critics who say his policies harm the poor.

“During our time there has not been a single corruption allegation against us,” he said “We believe in treating everyone equally and taking the country on the path of development.“