Menendez’s chief of staff, Danny O’Brien, said the senator maintains his innocence and has taken responsibility for his error in failing to pay for his charter flights with Melgen, a longtime friend.

“The Senator realized it was an oversight, that it was sloppy,’’ O’Brien said. “It was junior varsity at best.’’

By reimbursing Melgen for the flights, Menendez effectively undercut a standing ethics complaint and the possibility he’d face something more serious: a possible federal charge.

“It’s technically a federal crime to not report gifts on a federal financial-disclosure form,” said Melanie Sloan, executive director of CREW.

The flights, however, would no longer be considered a gift because Menendez just paid for them. And it doesn’t matter that he took the then-freebie flights more than two years ago, in 2010.

Menendez would have been required to disclose the flights on his federal financial disclosure forms if the trips were considered gifts that he didn’t pay for.

Menendez’s office said the senator could have claimed the flights as gifts, however, because Melgen is a longtime friend and would qualify under a type of friendship exemption concerning gifts to sitting members of Congress.

Melgen’s a big supporter of the senator, too.

Melgen and his family contributed $33,700 to Menendez, and another $60,400 to the Democratic Senatorial Campaign Committee, which Menendez chaired from 2009 to 2011, when he took the trips.

About that time, Melgen used a West Palm Beach-based company to buy a 50 percent share in ICSSI.

For more than a decade, the company had held a port-security contract in the Dominican Republic. But it had never gone into effect because of protests from a former Customs agency director who said the deal unfairly benefited the company.

Meanwhile, the country’s business community also complained that the contract would increase fees and hurt trade.

Last month, the president of country’s shipping association told Hoy newspaper that the industry couldn’t afford to pay additional fees to X-ray cargo.

“The problem here is that we’re looking at a contract that looks unfair because it was signed several administrations ago and that it essentially creates a monopoly in all the ports,’’ said association president Teddy Heinsen.

But Melgen’s supporters in the Dominican Republic charge that opposition to the deal is also rooted in the drug trade. The more port security and the more X-rays, they say, the more authorities will be aided in combating drug trafficking.

Menendez, when he spoke at the July hearing in Washington, didn’t sound more concerned about ICSSI than other companies. He mentioned a road builder by name that he thought was being unfairly treated. And he also brought up a dispute involving textiles.