The billionaire heirs to the Walmart fortune have thrown in $1 million to help embattled Republicans keep control of the New York state Senate, new campaign filings show.

Alice and Jim Walton each donated $500,000 to New Yorkers for a Balanced Albany, the pro-charter group that is spending massive sums to help Republican candidate Julie Killian defeat Democratic Assemblywoman Shelley Mayer in a special election Tuesday to fill a vacant Senate seat in Westchester.

The seat was previously held by Democrat George Latimer, who was elected Westchester County executive last fall.

If Killian wins, Republicans would maintain their razor-thin 32-seat majority in the 63-seat Senate.

If Mayer wins, Democrats would have a 32-seat numerical majority. But they would only be able to control the chamber if conservative Brooklyn Sen. Simcha Felder, who caucuses with the GOP, flips sides.

The financial intervention by the Walton family reveals the stakes in the elections.

The Senate GOP majority has been staunch supporters of privately managed and publicly funded charter schools, at a time when Mayor Bill de Blasio has sought to limit their expansion in the city. The Walton family has contributed millions of dollars to the charter school movement.

Most charter schools, like Walmart, have a non-unionized workforce. And that’s the rub with the political left and teachers’ unions in New York.

Big Labor and their Democratic allies have long considered Walmart an enemy. They’ve repeatedly beat back attempts by Walmart to open stores in New York City, claiming the nation’s retail giant is anti-labor.

More than $3.5 million has already been spent on the Killian-Mayer race.

The Balanced Albany group is spending much of its money on ads blasting Mayer.