TRENTON — Veteran prosecutors and defense attorneys said Friday the government faces a formidable challenge in building a public corruption case against Sen. Robert Menendez, who is reportedly under investigation by a federal grand jury in Miami over his relationship with a wealthy donor.

Menendez has intervened with federal agencies on several occasions to advance the business interests of Salomon Melgen, a Florida eye doctor who is one of his biggest supporters. Records show the senator’s help was often followed by tens of thousands of dollars in campaign contributions from Melgen.

“You must show a specific connection between whatever thing of value you are alleging was given and an official act,” Stanley Brand, a veteran Washington defense lawyer on public corruption cases, said in a telephone interview. “The (Justice) Department has shied away from it because it’s so difficult.”

In January, FBI agents raided Melgen’s offices in West Palm Beach and left with dozens of boxes of documents, although they have never said what they were investigating. But the incident set off a series of revelations about the decades-long friendship between the two men.

Now a federal grand jury is looking into the ties between Menendez and Melgen, who has provided Menendez with round-trip flights to his home in the Dominican Republic, the Washington Post reported Thursday. The FBI and the U.S. Attorney’s Office in Miami declined to comment.

Brand and other experts said the close friendship between Menendez and Melgen will make criminal charges hard to prove beyond a reasonable doubt because the two men can claim their personal relationship was a two-way street rather than a sinister arrangement.

“I’ve been around this stuff for 40 years,” Brand said. “I can’t remember the department bringing a case based on what we know at this point.”

Menendez, a Democrat from Hudson County, has faced intense scrutiny since federal agents raided Melgen’s medical offices two months ago and news reports tied the investigation to anonymous and unsubstantiated allegations that the senator had sex with prostitutes, some underage, in the Dominican.

Menendez has vehemently denied the claims as a political conspiracy intended to thwart his rise through the Democratic Party ranks, and coinciding with assumption of the influential chairmanship of the Senate Foreign Relations Committee.

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One woman who accused the senator of sexual allegations has recanted her story.

Still, the flurry of attention revealed that in January, Menendez reimbursed Melgen $58,500 for two round-trip flights he had taken to the Dominican Republic on the doctor’s private jet more than two years earlier but had not disclosed, in violation of Senate rules.

Menendez said the lapse was an oversight during a busy travel schedule.

Beyond the trips, more substantive accusations of ethical misconduct have come to dog the veteran senator.

The New York Times reported that Menendez had publicly and privately interceded on behalf of Melgen over a disputed Dominican contract to provide cargo scanning that was worth up to $500 million.

The Washington Post reported that in 2009 and 2012 Menendez spoke with top federal health officials about a finding that Melgen had overbilled Medicare by nearly $9 million.

Records show that each time Menendez helped his friend, campaign donations flowed his way. For example, after Menendez brought up the billing dispute last June, Melgen wrote a $300,000 check to a super PAC that spent $582,500 to back Menendez during the fall Senate race, according to campaign records.

Paul Brubaker, a spokesman for Menendez, did not return calls for comment Friday.

But on Thursday, Brubaker told the Washington Post, "As we have said all along, we welcome any review because Senator Menendez's actions have always been appropriate, and we believe the facts will confirm that."

Experts contacted by The Star-Ledger cautioned that prosecutors often use grand juries as investigative tools, and their use does not necessarily mean they believe they have enough to indict or even intend to take it that far.

Daniel Stein, a former assistant U.S. attorney in the New York who has led many corruption prosecutions, said proving the thousands of dollars that Melgen gave Menendez and political organizations was in exchange for his help will take strong evidence like recordings or cooperating witnesses.

“The prosecution has to show there was an explicit quid pro quo between the donor and the recipient,” Stein said.

If investigators plan to build a case around benefits other than campaign contributions, he said, they would not have to show as clear a connection but would still have to prove an intention on Melgen’s behalf to provide something of value in return for an official action.

“You’d have to show whatever favorable actions the senator took were done because of the official action and not their friendship,” Stein said.

Ross Baker, a political science professor at Rutgers University who is an expert in congressional affairs, said Republicans would undoubtedly cause an uproar in the unlikely event Menendez was indicted.

“I suspect people would call for him to lose his chairmanship and to step down,” Baker said.

Star-Ledger staff writer Jarrett Renshaw contributed to this report.

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