New York’s powerful public employee unions are making huge, last-minute contributions to defeat a referendum on Tuesday’s ballot that asks voters whether they want to call a constitutional convention.

A once-in-a-generation opportunity to revise the state constitution could lead to changes long resisted by the state Legislature, such as term limits and measures to contain pay-to-play practices.

But Big Labor also wants to protect the status quo, including cherished pension benefits written into the state constitution.

The state teachers union — NYS United Teachers — pumped $444,000 into the anti-Con-Con campaign in the past few days alone, Board of Elections records show.

The Communication Workers of America donated $200,000.

The Nassau County and Suffolk County Patrolmen’s Benevolent Associations each donated $50,000 to scuttle the Con Con initiative, which by law must appear on the ballot every 20 years.

The American Federation of State, County and Municipal Employees added $50,000 and the Suffolk County Association of Municipal Employees gave $25,000.

The unions sent the money to an umbrella group called New Yorkers Against Corruption, which is spearheading the anti-Con Con campaign.

Even before these contributions came in, the anti-convention group had collected $1.8 million and spent over $1 million.

By comparison, the main proponents, the NY People’s Convention, has raised nearly $275,000 over the past month, spent $250,000 and had just $30,000 left.

The campaign to scuttle the Con-Con has clearly worked.

A Siena College Poll released on Wednesday found that voters oppose Con Con by a better than 2-1 margin, 57 to 25 percent.

“A lot more money has been spent to persuade voters to oppose the Con Con. It seems to be working,” said Siena College pollster Steven Greenberg.