At issue in the reimbursement dispute is Melgen's multiple use of individual vials for eye injections to treat macular degeneration. Federal auditors have said Melgen often billed the government three to four times for injections from a single vial, according to two federal officials and lawyers familiar with the case.

The government's Medicare program reimburses providers $2,000 for each vial, so Melgen was billing $6,000 to $8,000 for each vial.

Melgen's attorneys said the doctor was properly billing for treating four patients with medical injections, albeit from one vial.

After CMS ruled in 2008 that Melgen would have to repay the government, he and his legal representatives contacted Menendez's office, arguing that the finding was unfair, the senator's aides said. Menendez's staff members had several conversations with agency officials to learn more about the billing rules and the details of Melgen's case in particular, the aides said.

In July 2009, Menendez called Jonathan Blum, the Medicare director at CMS, to express concern, the aides said. Menendez brought up Melgen's case, they said, in the context of broader concerns about the guidelines.

Then, in June 2012, Menendez raised Melgen's case again at a meeting with CMS Acting Administrator Marilyn Tavenner, aides recounted.

Melgen has been a friend and political supporter of Menendez's for many years. Last year, his practice gave $700,000 to Majority PAC, a super political action committee set up to fund Democratic candidates for Senate.

Aided by Melgen's donation, the super PAC became the largest outside political committee contributing to Menendez's re-election, spending more than $582,000 on his behalf.

There was the $295,000 he received from biotech companies and hospital PACs. The senator later authored tax breaks for biotech companies.

There was the $825,000 he received from contractors and landowners who benefited from the development of a Hudson County light rail system. The senator fought for federal money to get it built.

There was the $13,700 contributed to Menendez by a Hackensack company that makes artificial knees.

The senator urged the Food and Drug Administration to license the product. And the company hired his former chief of staff as a lobbyist.

Some of those connections include the major overhaul of health insurance known as Obamacare, which included provisions Menendez authored that provided tax breaks for biotechnology companies and higher Medicare reimbursements for urban hospitals. And biotech company PACs and employees gave at least $95,000 toward his reelection last year, while hospital PACs and employees gave more than $200,000.

Menendez also introduced a bill to relax rules on foreign investment in commercial real estate, and the head of the Real Estate Roundtable raised $32,000 in contributions for him last spring, while the National Association of Realtors "bundled" an additional $21,000 in late 2011.

And he wrote a letter to the Federal Reserve supporting the sale of a troubled Elizabeth bank whose executives had contributed more than $50,000 over his career. He reintroduced a bill in July 2008 to legalize Internet poker, and deposited nearly $16,000 in contributions from professional poker players a month later.