Sen. Robert Menendez (D-N.J.) in Garwood, N.J., in 2015. Menendez’s lawyers appeared in federal court Monday to press his appeal of the federal corruption case against him.

PHILADELPHIA — Attorneys for U.S. Sen. Robert Menendez (D-N.J.) sought to persuade a federal appeals court here Monday to throw out the public corruption case against the legislator before it ever gets to trial, arguing that he is protected by a law that gives members of Congress special legal consideration.

Menendez, 62, whose Senate term does not end until 2019, did not appear at the U.S. Court of Appeals for the 3rd Circuit, where a three-judge panel peppered his defense team and prosecutors with skeptical questions. Representatives said he was traveling to D.C. The hearing was the latest in what has been an aggressive and multi-faceted effort by Menendez’s attorneys to short-circuit the case, and though the arguments were complicated and technical, the discussion once again thrusts the senator into the spotlight.

Menendez was accused last year of helping a Florida ophthalmologist with government business in exchange for campaign contributions and posh getaways. He has pleaded not guilty and has vowed to fight for his exoneration.

Menendez is arguing that he is protected by the constitution’s “speech or debate clause,” which prohibits prosecutions of congressmen for legislative activity. Prosecutors assert that Menendez has not been charged for “legislative acts,” but rather, for using the power that comes with his office to help a friend and benefactor.

A U.S. District Court judge in September rejected Menendez’s bid to dismiss the charges on “speech or debate” grounds, and he soon appealed. Two of the judges hearing his case Monday were appointed by Republican presidents — Kent A. Jordan by George W. Bush and Anthony J. Scirica by Ronald Reagan. The third, Thomas L. Ambro, was nominated by Bill Clinton.

Menendez’s friend and benefactor, Salomon Melgen, is also charged in the case with bribery and is awaiting trial.

The debate centers on different interpretations of Menendez’s actions and whether they were legislative. In the prosecutors’ telling, Menendez tried to influence immigration-visa proceedings for Melgen’s girlfriends; assisted Melgen in a dispute with the Center for Medicare and Medicaid Services, which claimed Melgen had overbilled by $8.9 million; and advocated for action to help Melgen make money from a port security contract in the Dominican Republic. None of Menendez’s conduct, they said, was truly legislative; he merely used his position to press those in other branches of government to help his friend.

In exchange, prosecutors said, Melgen lavished Menendez with first-class or private-jet flights, use of a Caribbean villa, access to an exclusive Dominican resort and a stay at a luxury hotel in Paris. Melgen also provided Menendez expensive meals, golf outings and hundreds of thousands of dollars to help his 2012 Senate campaign — often timed closely to help that Menendez gave Melgen, prosecutors alleged.

“The indictment does not allege legislative acts, and the Speech or Debate Clause does not immunize Appellant from its charges,” prosecutors wrote in a court filing, adding that the clause was not “a blanket protection shielding members of the Legislative Branch from any Executive Branch prosecution.”

Menendez’s attorneys argued that Menendez was often pursuing broader policy goals or oversight responsibilities, even when he was trying to influence those in other branches of government, and prosecutors had no evidence tying Menendez’s actions explicitly to Melgen’s largess. They said the lower court had interpreted what was protected by the speech or debate clause too narrowly, and its ruling “seriously undermines separation of powers.” The clause is intended to enhance the independence of the legislature and prevent its members from being intimidated by the executive or judicial branches of government.

“There has never been a bribery case that has been made by the Justice Department where the quo for the quid was the legislative act itself,” Menendez defense attorney Abbe Lowell said in court.

It was difficult to ascertain which way the judges were leaning. Jordan pressed Lowell on whether the actions Menendez took affected anyone but his friend, and the judge said at one point, “You seem to be attacking a little bit of a straw man here, Mr. Lowell.” Scirica said he was “having a hard time understanding why this is true legislative oversight” — which is critical, because if the judges thought Menendez was exercising legislative oversight, he would be protected. Ambro asked Lowell about how judges should handle conduct that fell in the “middle” between legislative and non-legislative activity.

But the judges also asked probing — albeit fewer — questions of prosecutors. Jordan, for example, asked Justice Department lawyer Peter Koski whether the judges might risk “opening up the gates” to civil lawsuits against legislators for taking action that might leave a company unhappy. Koski said he was not asking the judges to create new law and that Menendez’s attorneys were failing to “meaningfully engage the indictment.”

It is unclear when the appeals court might rule, though if they allow the case to go forward, Menendez will still be allowed to press his innocence at trial, a date for which has not been set. Lowell declined to comment after the hearing.