The federal indictment against New Jersey Sen. Bob Menendez turns in part on prosecutors' ability to show that the gifts and favors at the center of the case amount to bribery rather than reflections of a longtime friendship between the lawmaker and the donor.

That burden of proof is among the issues that make public corruption cases more complicated for the government than they would appear from the pages of an indictment.

"It relies very heavily on quid pro quo — giving this official action for that thing of value. It's not a fuzzy relationship, it's not an iffy relationship," said Robert Walker, a former Justice Department corruption prosecutor who also served as chief counsel of the House and Senate ethics committees. "It can be difficult to prove quid pro quo."

Another potential hurdle for prosecutors is a protection the Constitution gives members of Congress for their legislative acts, an issue that's repeatedly surfaced in other cases involving federal lawmakers.

A grand jury indictment issued Wednesday in Newark charges Menendez with accepting a series of gifts, including flights aboard a luxury jet and a Paris vacation, from Salomon Melgen, a wealthy Florida eye doctor, political donor and friend of two decades.

Prosecutors say Menendez, in exchange for gifts and campaign contributions, acted to advance Melgen's business interests, including by intervening in a Medicare billing dispute worth millions of dollars.

Menendez defiantly asserted his innocence in a public appearance hours after the indictment, predicted he would be vindicated and said Justice Department prosecutors "don't know the difference between friendship and corruption." On Thursday, he pleaded not guilty to charges including bribery, conspiracy and making false statements. He remains free but had to surrender his passport.

"Prosecutors at the Justice Department often get it wrong," said his lawyer, Abbe Lowell, after Thursday's arraignment. "These charges are the latest instance of that."

As the case proceeds, both sides will present dramatically different portraits of Menendez's relationship with Melgen.

In the government's telling, the lavish travel and vacation were given with the corrupt intent to spur specific acts on Melgen's behalf. But defense lawyers are likely to cast the gifts as tokens exchanged between men who've shared a close bond for 20 years.

And what prosecutors call unlawful favors will invariably be characterized by Menendez's defense lawyers as actions that fall within a senator's ordinary duties — and that were done for the right reasons.

"Because this was a real friendship and not a corrupt relationship — and because Senator Menendez's actions were proper — this case too will become another of those mistaken cases that should have never been brought," Lowell said.

That close relationship will likely not only be central to the defense but also stands apart from the one in last year's corruption case of ex-Virginia Gov. Bob McDonnell and wife Maureen, where a businessman who showered them with gifts became intertwined in their lives only after they had reached the executive mansion.

"I suspect this will be a hard-fought argument on both sides — whether these gifts were the result of a close long-term family friendship" or something more nefarious, said Scott Fredericksen, a Washington defense lawyer and former federal prosecutor.

A separate potential looming legal issue is the Constitution's "speech or debate" clause, which protects elected officials from being questioned by prosecutors, among others, about their legislative work.

Courts have historically understood the clause to shield members of Congress from legal scrutiny for votes and other core legislative acts. The provision's been invoked with varying degrees of success by members of Congress seeking protection from prosecution, or law enforcement searches, for legislative acts.

Edward Loya, a Los Angeles lawyer and former federal prosecutor, said that despite the legal complexities of public corruption cases, it will likely be hard for Menendez to prove that the actions he took were part of the ordinary legislative process — or that he wasn't "going out on a limb" for Melgen by contacting executive agency heads on the doctor's behalf, in certain instances.

"The official acts that Sen. Menendez engaged in don't have a clear explanation other than his apparent interest in trying to help a personal friend who was providing him benefits and things of value," added Loya, a former member of the Justice Department's Public Integrity Section, an anti-corruption unit handling the prosecution.

The section has had a mixed track record against high-profile public figures in recent years, a reflection of some of the inherent challenges of the cases.

The Menendez case will be the first against a sitting senator since the botched prosecution of the late Alaska Sen. Ted Stevens, which the Justice Department ultimately dismissed after admitting that it failed to turn over evidence.

Prosecutors from the section later lost a campaign finance case against Sen. John Edwards — another Lowell client — but helped secure corruption verdicts last year against the McDonnells in Virginia.

Outside court Thursday, Lowell cited both the Stevens and Edwards cases as examples of what he described as Justice Department mistakes — a public salvo in a high-stakes, high-profile legal fight.

"This is shaping up as a quite good battle," Fredericksen said.

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