WASHINGTON (AP) -- An ethical cloud is following Sen. Robert Menendez in Washington, even as he has leading roles in some of Congress' most important policy debates.
The New Jersey Democrat heads the Senate's Foreign Affairs Committee at a time when tensions with Syria, Iran and North Korea are rising. At home, the Cuban-American lawmaker is at the center of the immigration overhaul fight that's captivating Capitol Hill.
All the while, a Senate Ethics Committee investigation into Menendez's conduct persists, with many of the same questions about politics, money and influence that have dogged members of Congress through history: When does helping a supporter become a quid pro quo?
At issue is Menendez's relationship with Dr. Salomon Melgen, a Florida ophthalmologist under FBI investigation.
The two men have long and deep ties, and their relationship bears similarities to past congressional scandals, experts said.
Menendez has acknowledged several instances when actions he or his office has taken appear to have benefited Melgen. Menendez insists he's done nothing wrong.
"No one has bought me, No. 1. No one. Ever," Menendez has said. "In the 20 years I've been in Congress, never has it been suggested that that could even be possible. Never in 40 years of public life. So I'm not going to reach this moment in my life to make that a possibility."
It's a familiar Washington dance.
From the "Keating Five" scandal, one of the more celebrated congressional ethics cases, to more recent investigations, members of Congress have regularly run into trouble over the question of when helping a supporter crosses a line into impropriety.
"Most senators are doing this and you rarely get nailed for it," said Richard Painter, chief ethics lawyer under President George W. Bush and now a law professor at the University of Minnesota.
Menendez has run into trouble because "he didn't dot the i's and cross the t's. That allows (the Senate Ethics Committee) to open up a file and investigate."
Painter said that is similar to how past ethical scandals have unfolded.
During the Keating Five scandal, Sen. John McCain, R-Ariz., drew attention for flights he took on the plane of Charles Keating Jr., a top donor. Menendez's flights on Melgen's plane created a similar stir.
Ethical experts said Menendez's publicized actions are similar to those that have brought censure in the past.
"It's clear to me he crossed the line," said Melanie Sloan, a former federal prosecutor and the executive director of Citizens for Responsibility and Ethics in Washington, a watchdog group.
Sloan said Menendez's advocacy for Melgen on numerous fronts, particularly with federal regulators, is what makes his situation stand out as ethically questionable.
Lawmakers consistently find themselves on slippery terrain by helping donors, said Sheila Krumholz, executive director of the nonpartisan Center for Responsive Politics in Washington. She said that appeared to be the case with Menendez, too.
"We certainly have run across our fair share of scandals inspired by or really cemented by actions taken on behalf of donors," Krumholz said. "Often, too often, there's a nexus between top donors giving significant amounts of money and that money amounting to a significant return on investment."
She added, "Not a Congress goes by without there being some question of improper actions being taken."
Melgen is Menendez's top political donor. Last year, Melgen's practice gave $700,000 to Majority PAC, a super PAC set up to fund Democratic candidates for the 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 the senator's behalf.
In January, Menendez reimbursed the doctor $58,000 for flights to the Dominican Republic aboard his private plane. Painter said Menendez's flights with Melgen were a red flag that elevated his case.
"It becomes a question of what other stuff is there," he said.
Menendez has acknowledged at least three areas where his official work appeared to serve the interests of Melgen, though he has steadfastly denied wrongdoing.
A staffer in Menendez's office sent emails to the Homeland Security Department in January questioning potential U.S. donations of cargo-screening equipment to the government of the Dominican Republic.
At the time, donated screening equipment could have jeopardized a lucrative port security contract for a company owned by Melgen. The government told Menendez's office that it had no plans to donate equipment. That came six months after Menendez himself raised concerns about the Dominican government's port security in a Senate hearing with senior officials from the State and Commerce departments.
Menendez sponsored legislation with incentives for natural gas vehicle conversions that could have benefited a company that Melgen was invested in called Gaseous Fuel Systems Corp. of Weston, Fla.
In another instance, Menendez has acknowledged that his office contacted U.S. health agencies in a way that would help Melgen. Menendez said he contacted the U.S. Centers for Medicare & Medicaid Services to ask about billing practices and policies. The contacts came during a dispute between CMS and Melgen over millions of dollars in Medicare payments.
It's Menendez's interactions with health officials that draw the most direct comparisons to the Keating Five case, Sloan said.
"I think we're always going to see stuff like this," Sloan said. "But it also opens a window into the way Congress operates. There may not be a quid pro quo in every case, but they're very often taking actions for top donors and that's why people are cynical."
The Senate Ethics Committee also investigated the Keating Five case, wrapping up its work in 1991. It centered on five senators — four Democrats and one Republican, McCain.
All took contributions from Keating, a real estate investor and savings and loan owner who became the face of the savings and loan crisis that cost many investors their life savings. Keating's Lincoln Savings and Loan was among many institutions that failed.
McCain took $112,000 in donations from Keating, his family and associates for early political campaigns. Like Menendez, McCain also flew on a plane owned by his donor, traveling to the Bahamas among other places.
McCain eventually repaid all of the donations tied to Keating to the U.S. Treasury and reimbursed Keating for his travel. Again like Menendez, McCain said at the time he believed that Keating had previously been repaid for his travel, but he had not been.
An ethics probe was triggered after McCain and the other senators advocated for Keating with financial regulators in 1987, as they investigated Keating and considered referring criminal charges to the Justice Department. Keating eventually went to prison.
The Senate Ethics Committee found three senators had interfered significantly and inappropriately with federal regulators. McCain was cleared of acting improperly, but the committee said he had "exercised poor judgment in intervening with the regulators."
None of the five senators involved resigned, and two — John Glenn, D-Ohio, and McCain — ran successful re-election campaigns.
Writing about the case in his 2002 book "Worth the Fighting For," McCain said he vowed to never again intervene in regulatory decisions if "such interventions could be construed, rightly or wrongly, as done solely or primarily for the benefit of a major financial supporter of my campaigns."
In interviews about Melgen, Menendez has sought to distinguish between questioning policies he said he finds questionable and taking actions to benefit his top contributor.
"The fact that someone is a donor does not do away with the right or the opportunity to consider whether something is correct or incorrect, to ask questions, raise concerns," Menendez has said.