The striking United Auto Workers union has teamed up with an ex-con killer and rapper from Flint, Mich., to bash Detroit’s auto giants.

The protest song “Big 3” by ex-con Jemini Smith, who raps under the moniker Gemini Ramone The OG, was used to kickstart Friday’s media address from UAW President Shawn Fain — a move that may have been calculated to help Fain connect with the many black members in the 25,000-strong union currently picketing the automakers, a tipster told The Post.

In the clip, Smith — who was convicted of second-degree murder for killing 16-year-old Vartina Hamlin in 1988 and then busted with a stolen gun while on probation in 2013 — rails against the automakers, according to Michigan Live.

“Stellantis, GM and the other one is Ford, they all build cars that their workers can’t afford,” raps Smith, who is in his early 50s, in the clip.

“Make that make sense cause for me that don’t at all, we struggle to survive while they live high off the hog,” he says.

“Now I’m line doing a picket with the sign, but treating us fair eliminates this waste of time,” raps Smith, who is from the vehicle-manufacturing stronghold. “Tell Shawn Fain I really love his strategy, and tell the Big Three this never ever had to be.”

The UAW did not immediately return a Post request for comment about Smith’s ties to the protest — including appropriateness of using a convicted killer as a spokesman.

Smith also did not immediately respond to a Post interview request on his personal Facebook profile, where he was promoting his protest song.

For the first time in the powerful union’s 88-year history, its members are simultaneously striking at plants of all three major US auto makers, seeking better wages and benefits.

The strike entered its third week Friday and expanded to Ford’s Chicago assembly plant and GM’s Lansing, Mich., assembly plant, according to Fain.

About 17% of the union’s 146,000 members are now on the picket line, trying to strategically play the companies against each other.

Stellantis, Chrysler’s parent company, was spared another walkout Friday as it gave last-minute concessions on cost-of-living allowance payments and employees’ right to strike.

The strike began Sept. 15 as workers walked out of one plant from each of the Big Three.

A week later, workers at GM and Stellantis distribution facilities in 20 states followed suit. Ford was spared a further escalation until negotiations stalled.

Picketers receive $500 a day from the union, which is looking to boost pay and eliminate a two-tier wage system that leaves newer workers with significantly lower salaries than their peers.

Smith was sentenced to 25 to 50 years behind bars for the murder of Hamlin after he and two other men opened fire on more than 100 people standing outside a Flint house party, according to the local outlet.

Hamlin, who was described as good student and member of the school’s track team, was not the intended target and died after being shot in the back by Smith’s .22-caliber “machine gun.”

He was eligible for parole in 2012, according to the article.