The menacing letter sent out to a million New York Democrats to bully them into voting was written by a top campaign strategist who was appointed by Gov. Andrew Cuomo himself, sources told The Post.

The revelation comes as a “circular firing squad” of finger-pointing has erupted over who is responsible for the letter, which was sent out last week by the New York State Democratic Committee and caused “panic” in the Cuomo camp, sources said.

The letter warned voters that the party would be keeping tabs on who goes to the polls on Tuesday.

“Who you vote for is your secret. But whether or not you vote is public record,” the letter says.

“We will be reviewing voting records . . . to determine whether you joined your neighbors who voted in 2014.”

It ends with the ominous line: “If you do not vote this year, we will be interested to hear why not.”

Neal Kwatra, appointed by Cuomo to be the state Democratic Party’s chief campaign strategist, was identified by two key Democratic insiders as the “author” of the mailer.

“It was the brainchild of Neal Kwatra,” said one of the insiders. “He wrote the letter and sent it out, and now other Cuomo people are trying to distance themselves from it.”

The intimidating letter has caused friction inside the Democratic campaign just days before Cuomo faces off against Republican Rob Astorino at the polls.

“There’s infighting going on, a definite circular firing squad over the mailer and a sense of panic over it in Cuomo’s camp right now,” the insider continued.

The governor claimed Friday that he was not aware of the campaign mailer.

But a senior Democrat called Cuomo’s claim “laughable” and noted, “Nothing like this could possibly be done without Andrew’s OK.”

Kwatra gave a weak defense of the letter over social media, comparing it to a Republican letter being used in a Senate campaign in North Carolina.

The North Carolina letter, however, simply informed voters that their neighbors have voted early while they have not.

The New York letter told recipients that state Democrats knew they hadn’t voted in the past, that their voting records were public and that they were under scrutiny.

A nationally prominent New York-based Republican pollster strongly denied that such threatening letters had become common in politics.

“I’ve never seen anything like that letter in my life. It’s outrageous, and it’s creepy,” said James McLaughlin, a partner in McLaugh¬lin & Associates, which works for Astorino.

“This sounds like the Democrats are saying, ‘We’re watching you, and if you don’t turn out to vote, the brown shirts are going to be knocking at your door.’ ”

Many Democrats are furious that Cuomo and his campaign operatives, including party spokesman Peter Kauffmann, claimed the letter was merely part of a “nationwide Democratic response to traditional Republican voter-suppression efforts.’’

“That’s laughable,” a prominent party activist involved in national Democratic campaign efforts told The Post.

“No one I know at the Democratic National Committee has ever seen anything like this letter, and it’s an act of cowardice to say that it’s part of a coordinated effort.”

Several Democrats told The Post the letter was part of Cuomo’s effort to turn out a larger Democratic vote amid widespread concerns of voter apathy.

Meanwhile, the Staten Island Republican Party said Sunday that it was flooded with phone calls from people who were scared that their ballots would be tampered with because they are registered Democrats.

“Some senior citizens are worried that their absentee ballots may be at risk because they cast their votes for Republicans,” said the borough’s Republican chairman, John Antoniello.

“They feel like Big Brother may be watching them. It’s the absolute wrong way to win elections.”