​Fellow Democrats don’t know what Gov. ​Andrew ​ Cuomo was thinking when he launched his sharpest barb yet at Mayor ​Bill ​de Blasio — over the issue of government corruption.

In an extraordinary broadside, senior Cuomo campaign ​adviser Sarah Paden on Monday told The Post that the governor “doesn’t know why the prosecutors gave the mayor a pass on his bribery​,” referring to uncharged allegations by crooked restaurateur Harendra Singh.​

The blistering attack on de Blasio’s integrity came less than two weeks after former top Cuomo aide Joseph Percoco was found guilty of corruption charges — and ahead of the corruption trial involving Cuomo’s signature “Buffalo Billion” economic-development program.

Former Westchester Assemblyman Richard Brodksy called the move “nutty.”

“Everyone is entitled to an opinion. But you’ve got to scratch your head and wonder: What is the purpose here? It doesn’t exactly advance the governor’s record,” he said.

A top Democratic consultant said that “Cuomo’s playing with fire,” and a Democratic operative with close ties to City Hall called Paden’s statement “political malpractice.”

But Paden doubled down Tuesday, saying: “We just said what everyone else in New York was already thinking, saying and writing.”

De Blasio spokesman Eric Phillips said: “Everyone understands Gov. Cuomo is going through a tough time right now, so we’ll decline to comment on his deflection tactics or obsession with the mayor.”