Gov. Andrew Cuomo will call a special election for the 11 legislative vacancies in the Senate and Assembly for April 24 in the coming days, a source with knowledge of the plan said Tuesday.

The special election is key to the fulfillment of a deal that would unite Democrats in the state Senate, filling vacancies in two districts in the Bronx and Westchester County.

Under the agreement, the mainline Democratic conference in the Senate and Independent Democratic Conference would form a coalition, pending victories in those districts.

All eyes will likely be on the Westchester County race in what has been a district long sought by Republicans. It is expected to be one of the more costly special elections.

Democratic Assemblywoman Shelley Mayer is running for the Westchester County seat, while Republicans Dan Schorr and Sarmad Khojasteh are vying for the GOP nomination.

Some advocates had hoped the elections would be called before the state budget was agreed to, typically done by the end of March, the final month of the state’s fiscal year.

Should Democrats win both seats, the party would have a numerical majority in the chamber. Democrats would still have to work to win over Brooklyn Democratic Sen. Simcha Felder, a Brooklyn lawmaker who conferences with the Senate GOP and has said he would back whichever conferences offers the best arrangement for his constituents.

Senate Minority Leader Andrea Stewart-Cousins at an unrelated news conference before the news broke acknowledged she was frustrated that the seats had been empty for a month without a special election being called.

“We need to fill these seats as quickly as possible and I think we need to have a date,” she said. “It’s frustrating their isn’t a date and I hope one would be called.”

In a subsequent statement, Stewart-Cousins said she looked forward to Democrats winning the seats.

“I look forward to electing Democrats to both open seats and finally giving New York voters the functioning Senate Democratic Majority they have repeatedly voted into office, yet been denied. Luis Sepulveda, the Democratic nominee for SD-32, is a smart, effective leader and I look forward to him joining us in the State Senate,” she said.

“Shelley Mayer is an outstanding public servant who has been a true fighter for Westchester taxpayers and I look forward to her representing SD-37. We have the opportunity to make New York State the 9th ‘true blue’ state in the nation, and with all Democrats working together to achieve that goal, I know we will succeed.”

But that would also have the potential of upending the budget talks by changing the leadership in the closely divided state Senate.