Following being indicted on federal corruption charges, Sen. Robert Menendez (D-N.J.) says he is confident he will be "vindicated." (Reuters)

In April 2015, federal prosecutors dropped a bomb on one of Senate Democrats' leading foreign policy voices. They accused Sen. Bob Menendez (D-N.J.) of doling out political favors for one of his longtime friends in exchange for luxurious trips, lavish gifts and campaign cash.

Menendez, who was reelected to his second full term in 2012 and was the top Democrat on the Senate Foreign Relations committee at the time, has pleaded not guilty and said he has done nothing wrong.

His lawyers have spent the past year battling the FBI and Justice Department over whether federal prosecutors can actually investigate Menendez in the first place — a debate centered on a centuries-old constitutional clause that has the potential to redefine how the executive branch can look into the legislative branch. On Monday, Menendez's lawyers are asking a federal three-judge panel in Philadelphia to throw out some or all of the dozen or so bribery and corruption charges facing Menendez on the basis of a 1780s constitutional clause that they argue prevents scrutiny of lawmakers' official legislative activities.

What the judges decide later this year could have major ripple effects for how secure — or not — members of Congress feel in their day-to-day jobs from federal prosecution. Adding to the drama: The whole case has the potential to reach the Supreme Court. Here's what you need to know about this latest turn in a complicated corruption case, in three questions:

Remind me again of the case's details?

Sen. Robert Menendez (D-N.J.) was indicted on Wednesday on federal corruption charges. (Reuters)

This past spring, Menendez became just one of 12 U.S. senators indicted while in office. Proving corruption is a high bar in the U.S judicial system. In Menendez's case, prosecutors are trying to prove that Salomon Melgen, a Florida eye doctor, specifically received political favors from Menendez in exchange for giving Menendez something.

They are trying to show that Menendez intervened on behalf of Melgen twice — once to try to solve his friend's Medicare billings problem with the government and another trying to grease the wheels for Melgen's port cargo contract in the Dominican Republic. In exchange, prosecutors argue, Menendez accepted a series of trips and gifts over the years from his friend, including free private-jet flights, stays at resorts in Paris, trips to the Dominican Republic and more than $750,000 in campaign contributions.

"No ordinary constituent from New Jersey received the same treatment," the Justice Department said in an August filing. (In a sign of just how tough it is to prove bribery charges, in September, a federal judge threw out four bribery charges, saying Melgen's contributions to Menendez's legal defense fund didn't contain any explicit agreement of quid pro quo.)

Menendez has said his friend's trips and gifts were a natural result of his friendship and that he never inappropriately intervened on behalf of his friend. In what's become a nasty back and forth, lawyers have accused prosecutors of trying to dig up dirt on him — even his sex life. The investigation started after an anonymous tipster told the FBI and media outlets that Menendez was patronizing underage prostitutes in one of his trips to the Dominican Republic. Those allegations appear to be fabricated, wrote The Post's Carol Leonnig in August, but they've given the defense leverage with which to argue that the Justice Department will "stop at nothing" to try to put Menendez behind bars.

So what's going on Monday?

During a news conference, March 6, 2015, Sen. Robert Menendez (D-N.J.) said he had no intention of resigning during the trial phase of the corruption investigation against him. (Reuters)

Three federal appellate court judges will be hearing arguments from both sides about something much broader than the two friends' alleged quid pro quo. The judges will be considering whether the executive branch, in the form of the Justice Department, overstepped its constitutional bounds by investigating and charging Menendez in the first place.

Menendez's lawyers are arguing that the senator was doing official legislative business in advancing his constituent's interests in Washington, and the Constitution's "speech or debate" clause prevents lawmakers' legislative acts from being scrutinized by the other two branches of government — except for "Treason, Felony and Breach of Peace." Menendez was engaged in none of that, they say, and they're asking the judges to throw out at least some of the dozen or so counts of bribery and corruption on the claim that federal prosecutors unconstitutionally investigated Menendez.

Federal prosecutors argue that the "speech or debate" clause doesn't apply if lawmakers are trying to influence the other branches of government, and they say they can prove in court that Menendez was trying to do just that.

What the judges decide later this year could go to trial, possibly right around the time of the November presidential elections. Depending on how the judges rule later this year, the case and its applications to the "speech or debate" clause could also end up at the Supreme Court.

What happens next?

That's an open-ended question. Over the years, courts have expanded the protections lawmakers have from investigation to go beyond what they say or do on the House or Senate floor, reports the Associated Press's David Porter. But Lee Vartan, a former assistant U.S. attorney in New Jersey, told Porter that a case as specific as this is "uncharted territory."

And that's what makes this case and its potential legal ramifications so appealing to analyze. It is a high-profile case litigating a not-widely-known constitutional clause. What the judges decide could set the precedent for how protected —or not — lawmakers feel they are from investigation in the future.