In telling his own story, Menendez likes to talk about his scrappy roots. The first paragraph of the biography on his official Senate website notes that he is the son of immigrants who grew up in a tenement in Union City, N.J. Menendez, 59, rose from the local school board to mayor to state legislator to House member to U.S. senator. He recently became the chairman of the Senate Foreign Relations Committee.

As Menendez has accumulated political power, however, he has not accumulated great wealth. His 2011 financial disclosure, filed last May, lists checking and savings accounts that held somewhere between $66,003 and $165,000. (Congressional disclosure forms list assets in ranges.) He also owned stock in a single company, Metropolitan Life Insurance, worth between $1,001 and $15,000. A rental property he owned in Union City generated between $15,001 and $50,000 in income in 2011. (The property itself was valued at between a quarter-million and a half-million dollars, with Me-nendez still owing somewhere between $50,001 and $100,000 on the mortgage, the disclosure report shows.)

It would appear to be a comfortable enough living, especially with a senator's annual $174,000 salary. But not so comfortable that cutting a $58,500 check couldn't have a profound impact.

Menendez's office declined to comment on the senator's personal finances or the details of how he managed to pay back the $58,500 for the two flights.

On Monday, Menendez told CNN that paying for the flights simply "fell through the cracks."

"When it came to my attention that payment had not taken place, I personally paid for them in order to meet my obligation," he said.

Government watchdogs are dubious. They say Menendez's financial situation adds fuel to questions about his motives and whether the free flights he accepted were a simple oversight.

"For a senator that's not a Rockefeller, that's real money," said Meredith McGehee, policy director for the nonpartisan Campaign Legal Center. "It kind of makes you wonder .... If he knew in advance that the trips were going to cost him $60,000, would he have done it?"

In the years after the Jack Abramoff scandal, which involved trips abroad for politicians, McGehee said it "stretches credibility" that Menendez was unaware he was receiving a gift while boarding a private flight to a Caribbean island. "You're about to walk on a private plane, and you're a public official -- and that doesn't occur to you?" she said.

Melanie Sloan, executive director of Citizens for Responsibility and Ethics in Washington, another watchdog group, was even less charitable.

"He waited until he was caught to pay them back," she said. "If you rob a bank -- and you're caught -- you don't say, 'Take the money back and forget about it.' "

In a statement, Menendez's office defended the senator and said all flights had now been paid for. "Dr. Melgen has been a friend and political supporter of Senator Menendez for many years," the statement said. "Senator Menendez has traveled on Dr. Melgen's plane on three occasions, all of which have been paid for and reported appropriately." (The third trip was previously paid for and reported by the Democratic Senatorial Campaign Committee, which Menendez chaired in 2009 and 2010, as the senator was traveling to raise funds.)