(CNN) It's a case with allegations spanning seven years, multiple countries, and involving hundreds of thousands of dollars. At the center of it: a sitting US senator.

Prosecutors insist that the men's relationship was a "corrupt pact" in which Melgen showered Menendez with expensive vacations and campaign donations in exchange for the senator interceding on Melgen's behalf in several different disputes with government officials -- allegations the two vigorously deny.

The case also puts the Justice Department's Public Integrity section back in the spotlight, as Menendez is the first major bribery trial against a sitting US senator in nine years.

What is the case about?

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The allegations at the heart of the case begin soon after Menendez was elected to the Senate in 2006. Melgen, who defense attorneys claim was simply acting as a friend and supporter of the senator, allegedly began flying Menendez on a private jet to and from his Dominican Republic villa for weekend getaways free of charge -- all of which Menendez failed to report. Melgen also donated hundreds of thousands of dollars to the senator's campaign funds, legal defense funds and political action committees that supported the senator and paid for a pricey Paris hotel stay Menendez, according to court documents.

In exchange, prosecutors say, Menendez acted in his "official capacity" as a senator to help advance the personal and business interests of Melgen.

When an X-ray equipment business, owned by Melgen in the Dominican Republic, ran into problems with the country's government, prosecutors say that Menendez contacted the State Department and pressured them to "intervene with the Dominican government to resolve the dispute in Melgen's favor," according to a trial brief filed last week by prosecutors.

And when the Center for Medicare and Medicaid Services (CMS) slammed Melgen with an $8.9 million overbilling charge, Menendez allegedly contacted the head of the agency as well as then-Senate Majority Leader Harry Reid to intervene in the dispute after Melgen requested his help, prosecutors allege.

Menendez allegedly used his power to help Melgen in his personal life as well, prosecutors say. On at least three different occasions, Menendez allegedly helped Melgen's girlfriends obtain tourist visas to the United States to visit Melgen.

Together, Menendez and Melgen have pleaded not guilty to the 18 counts of fraud and bribery filed against them.

In 2013, after word of the federal investigation became public, Menendez paid back Melgen $58,000 for the 2010 plane trips calling his failure to properly disclose the flights an "oversight."

What are the legal issues involved?

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While much of the Justice Department's court filings allege Menendez enjoyed a "stream of benefits" from Melgen's coffers, legal experts say prosecutors will need to contend with a recent Supreme Court decision that raised the bar on what's now required to convict a public official of bribery.

Last year the court set aside the conviction of former Gov. Robert McDonnell because prosecutors had failed to properly instruct the jury on what type of "official act" is sufficient to support a conviction for bribery.

The justices ultimately agreed with McDonnell that merely setting up meetings, hosting dinners or calling another public official, without more, cannot constitute an "official act" -- a point Menendez's defense team has leveraged in pre-trial filings.

"McDonnell changed the landscape," says former federal prosecutor Adam Lurie, now a partner at Linklaters. "(Menendez's trial) is going to be the first case where both sides try to shape things in light of that decision."

But some legal experts say the McDonnell decision may not be what saves Menendez.

"Menendez's actions were much more substantial than McDonnell's," said Randall Eliason, a former chief of the public corruption section of the DC US Attorney's Office and now professor at George Washington University Law School, in a recent blog post

"Menendez did not simply arrange meetings for Melgen or introduce him to other officials. The senator himself attended various meetings and otherwise advocated for Melgen's interests. Unlike McDonnell, Menendez was actively engaged in trying to influence the outcome of the matters in question," he wrote.

Eliason predicts that the real crux of the case will instead come to down to whether prosecutors can prove Menendez had a "corrupt intent" -- an aspect of the trial made more challenging given that Melgen has not agreed to testify against the senator.

"This is a traditional bribery case and it all comes down to proving corrupt intent," Eliason told CNN in an interview. "The easiest way to prove a quid pro quo is to get the other side to testify -- prosecutors don't have that here."

CORRECTION: A previous version of this story misstated the law school where Eliason teaches.