They’re fighting to save Private Ryan — from jail.

Marines across the country are rushing to the rescue of former fellow leatherneck Ryan Jerome, who is facing prison time after trying to check his Indiana-registered handgun at the Empire State Building on Sept. 27.

In multiple blog and chat-group postings and letter-writing campaigns — some aimed directly at NYPD Commissioner Ray Kelly, a Marine Vietnam veteran — the Marines are urging that the charges be dropped against their “brother.”

“It is just overwhelming, the love and support I am receiving from my fellow Marines,” said Jerome, 28, a former private first class whose father and grandfather were Marines and who was a TOW gunner before his honorable discharge in 2005.

“It’s definitely emotionally devastating, looking at maybe facing 3 1/2 years prison,” he said in a phone call from his home in West Bend., Ind., referring to the mandatory minimum sentence he could face under New York’s strict gun laws.

The irony that someone so highly trained and trusted to keep America safe might be thrown into prison — for a .45-caliber Ruger that he had legally registered in his home state and that he was actually trying to check with authorities — has not been lost on his Marine brethren.

Boston lawyer and Marine vet Dave Bruce has organized an online campaign through leatherneck.com that will send its first 14 e-mails to Kelly, Mayor Bloomberg and DA Cyrus Vance today.

“The theme is not to have Mr. Jerome treated like a common thug. He deserves to be treated better,” Bruce explained.

The soft-spoken Jerome is a jeweler and had traveled to New York with $15,000 worth of gold he was taking to a Long Island refinery, said his lawyer, Mark Bederow.

Before coming, he had actually checked online with his phone to see whether his Indiana carry permit was reciprocal with New York, Bederow said. A cellphone glitch confused him.

The city has seen a spate of tourist gun arrests in recent months.