As more facts come out about the relationship between U.S. Sen. Robert Menendez, D-N.J., and his wealthy friend, Dr. Salomon E. Melgen, the situation gets more and more troubling.

Melgen, a Florida ophthalmologist, has been a major campaign contributor to Menendez and other Democratic senators.

He is also being investigated by the FBI and the U.S. Department of Health and Human Services, which says he overcharged Medicare $8.9 million.

Melgen owns a lavish home in a Dominican Republic oceanside resort, and he has flown Menendez there for vacations. Menendez never reported the last two trips, in 2010, as Senate ethics rules require, and did not reimburse Melgen for them until last month.

Menendez has said that he and Melgen are friends, and any suggestion that the campaign donations and free trips have influenced his behavior are off base.

But the Washington Post has reported that Menendez attempted to intercede in the doctor's billing dispute with federal health officials in 2009 and 2012, and that Melgen frequently dropped the senator's name in his dealings with fraud investigators.

If that's not unseemly enough, Menendez also has apparently pressured State Department and Commerce Department officials in support of a lucrative business venture Melgen is pursuing in the Dominican Republic. Melgen is part-owner of a company that has been trying to enforce a contract to provide port security in the Dominican Republic. That contract could be worth $500 million.

The New York Times reported in January that Menendez spoke to State Department officials about the contract and even questioned two administration officials about it at a Senate Foreign Relations subcommittee hearing he was presiding over.

On Sunday, the New York Times reported that Menendez's efforts to protect Melgen's contract extended to having his staff contact the Department of Homeland Security. In a January email, a Menendez aide attempted to discourage the department from donating to the Dominican Republic equipment that could be used to examine shipping containers for drugs and other contraband - an alternative to Melgen's contract.

Menendez became chairman of the powerful Foreign Relations Committee earlier this month, and prominent Democrats have been circling the wagons around him. Senate Majority Leader Harry Reid said, "I have confidence he did nothing wrong."

That may be the saddest part of this story. Apparently, in the Senate, behavior that would be unacceptable for a small-town mayor is considered nothing out of the ordinary. Campaigns require large amounts of cash, after all, and, the usual excuse goes, large donors don't buy votes, they only buy access.

But neither votes nor access should be for sale in the U.S. Senate.

How sad that we can assume 99 other senators are engaged in the same kind of friendly - and occasionally helpful - relationships with big donors.