A Queens Republican operative who is being investigated by the Manhattan District Attorney’s Office to find out what happened to $750,000 from Mayor Bloomberg’s campaign is nothing more than a common thief, a source in the mayor’s camp charged yesterday.

But pals of John Haggerty Jr. claim he’s been set up as a fall guy.

As The Post first reported in January, Haggerty set up a mysterious company, Special Election Operations LLC, to conduct what was supposed be a poll-watching operation on Election Day last year.

He has yet to explain what he did with the $750,000 delivered to him for that purpose through the state Independence Party.

A source in the Bloomberg camp said, “It looks like he took most of the money.”

But one pal said that because Haggerty wasn’t getting paid during the campaign, he was told he could keep whatever was left in the account he controlled after Election Day.

“They never gave him detailed instructions on how to spend the money,” insisted the pal, speaking of the Bloomberg campaign.

“They told him to do what he had to do. They knew exactly where the money was going. They never asked for any accounting.”

The friend added that Haggerty is prepared to spill everything he knows about the inner workings of Bloomberg’s operation if that’s what it takes to stay out of jail.

Sources said the mayor’s campaign is not a target of the probe.