Robert Menendez

U.S. Sen. Robert Menendez, shown here in a file photo. (Jonathan D. Salant | NJ Advance Media for NJ.com)

NEWARK -- When three federal appellate judges settle into their seats Monday to hear arguments in the corruption case against New Jersey Sen. Bob Menendez, they will be asked to delve deeply into an idea that, though little known outside legal circles, is as deeply embedded in the American DNA as the right to free speech.

How the court -- and a jury, if the case goes to trial -- places Menendez's actions in the context of the Constitution's "speech or debate" clause will play a significant role in whether the three-term Democratic senator is convicted on any or all of a dozen counts against him that include bribery, conspiracy and honest services fraud. Menendez has pleaded not guilty.

The clause dates to the 1780s and was written into the Constitution to fortify the separation of powers between the three branches of government: It protected members of Congress from having their legislative acts subjected to scrutiny by the other two branches, "in all Cases, except Treason, Felony and Breach of the Peace."

The clause has been invoked to shield members of Congress from testifying in court, and to exclude evidence from criminal trials.

In 2014, former Arizona Congressman Rick Renzi tried unsuccessfully to use the clause to reverse his money laundering and fraud convictions. In 2007, Rep. William Jefferson, D-La., used it to get some evidence obtained in a search of his Capitol Hill office excluded, but failed in his attempt to have his corruption indictment thrown out.

Last year, members of a House subcommittee investigating a former BP executive over the 2010 Gulf of Mexico oil spill invoked the clause to shield them from testifying in his criminal case.

To Menendez and his legal team, the indictment unsealed last spring against him and longtime friend Salomon Melgen, a Florida eye doctor charged with bribing Menendez for political favors, was an egregious example of the Justice Department's over-reaching into the legitimate affairs of a sitting lawmaker.

Prosecutors, on the other hand, have characterized Menendez as hiding behind the speech or debate clause to obscure the fact he was lobbying on behalf of his wealthy benefactor.

"The challenge is going to be showing that Menendez was doing something different or out of the ordinary for this particular constituent as opposed to what a senator or congressman does generally," said Lee Vartan, a former assistant U.S. attorney in New Jersey currently in private practice.

The trial was scheduled to begin last year, but the appeals process could push that to late this year.

Meetings Menendez arranged with four officials form the heart of the government's case, and the context and circumstances of those meetings have been parsed endlessly in voluminous court filings.

It boils down to this: Was Menendez lobbying for Melgen in a Medicare dispute or, as he argues, engaging in general legislative activity when he met with then-Health and Human Services Secretary Kathleen Sebelius, Sen. Tom Harkin and acting Medicare administrator Marilyn Tavenner?

Similarly, was he seeking to gain an advantage for Melgen's contract for port cargo screening equipment in the Dominican Republic when he met with an assistant secretary of state and later requested that U.S. Customs and Border Protection not donate security equipment to the Dominican government?

Over the years, courts have expanded the definition of what constitutes protected legislative acts beyond what takes place on the floor of the House or Senate, but there are limits. Without a wealth of Supreme Court cases that are directly on point, the appeals court judges could see fit to expand or shrink those limits, Vartan said.

"I think he has a shot, given the fact there's not a lot of case law," Vartan said. "It's uncharted territory to some degree."

The legal back-and-forth reflects the complicated issues surrounding the clause.

Both sides have used emails between the officials and their staffers to buttress their positions. Both also cited a 1994 ruling in the corruption case of former Rep. Joseph McDade, R-Pa., to justify whether Menendez's intentions for setting up the meetings can be explored.

The McDade judges also wrote, "acceptance of the bribe is the violation of the statute, not performance of the illegal promise," which could play a key role in Menendez's defense.

In a brief this month, Lowell argued the government "seeks to turn decades of precedent on its head by relying solely upon proof of the Senator's motive and legislative acts to prove bribery, without any evidence of an unlawful agreement between the Senator and his close personal friend, Dr. Melgen."

Prosecutors contend the constitutional clause doesn't cover efforts by members of Congress to influence the Executive Branch.

"Every act the indictment alleges Appellant and his 13 staffers took was designed and intended to influence the Executive Branch in Melgen's favor," prosecutors wrote this month. "The Speech or Debate Clause cannot save Appellant from its charges."