Barbara Howard: Federal prosecutors say they may have to drop a case against a pair of Boston city officials who face extortion charges in a trial slated to begin later this month. Kenneth Brissette and Timothy Sullivan have been on leave from City Hall since being arrested two years ago and accused of pressuring the organizers of the 2014 Boston Calling music festival into hiring union labor. Brissette and Sullivan allegedly threatened to withhold permits unless union workers were brought on, and festival organizers ended up complying. But federal prosecutors now say that the presiding judge's planned instructions to the jury may make it impossible to pursue their case. Here to explain is WGBH News legal analyst and Northeastern University law professor Daniel Medwed. Hi Daniel.

Daniel Medwed: Hi Barbara.

BH: So presiding over this case is Judge Leo Sorokin. And what is it that the prosecutors are saying the judge is getting wrong in his planned jury instructions?

DM: The judge says that in order to be guilty of extortion, you have to personally benefit from the activity, or a family member or an organization that you're affiliated with, while the prosecution's whole theory is that the union is benefiting. So basically, it's a very narrow definition of extortion that could make it very difficult for the prosecutors to obtain convictions.

BH: Well, Brissette and Sullivan — they may not benefit directly from pressuring Boston Calling to hire union labor, but could there be any indirect benefits?

DM: I think there is an indirect benefit, but the problem that I think Judge Sorokin is identifying stems from two other cases: a First Circuit case and a U.S. Supreme Court case that have really tightened up the definition of extortion and political corruption and made it much harder to prosecute these cases.

BH: Is it unusual for prosecutors to object this way to a judge's interpretation of the law and instructing the jury?

DM: Highly unusual. Jury instructions are often debated over the course of a trial and might become an issue on appeal. But I'm hard pressed to think of a similar case where there has been a pre-trial skirmish over jury instructions that’s resulted in a motion for reconsideration. This is highly unusual.

BH: Is there a chance that the judge — Judge Sorokin — changes his mind here and goes along with the broader interpretation of the law that the prosecutors want?

DM: There's always a chance, but most people don't change their mind once they've already made a firm decision in the absence of new information. So I highly doubt that he will change his mind. The other question is whether the First Circuit would agree to review this particular claim if, in fact, Judge Sorokin denies the motion.

BH: What would be needed for a guilty verdict?

DM: I think if this jury instruction stands, essentially the prosecutors — they've already suggested they might have to dismiss the case — they might not have the facts that would suggest that Brissette and Sullivan were pressuring the Boston Calling organizers to get a benefit for themselves or a family member. They would essentially have to prove a much closer personal benefit than the facts seem to suggest.

BH: For the layman, the question is, what's the harm in city officials asking that union labor be used?

DM: The concern is not so much that perhaps the city prefers union over non-union labor, though that's debatable whether that's a good thing or a bad thing. The concern is about city officials overreaching and potentially overstepping their boundaries by pressuring people to act in particular ways. I think that's the broader concern here — that potentially they could use their positions of influence to benefit certain groups and to disadvantage others.

BH: OK, thanks for joining us, Daniel.

DM: My pleasure, Barbara.

BH: That's WGBH News legal analyst and Northeastern University law professor Daniel Medwed explaining the case of Boston city officials Kenneth Brissette and Timothy Sullivan, who are set to go on trial later this month, facing extortion charges related to the hiring of union labor back in 2014 at the Boston Calling music festival. Prosecutors now say that they may have to drop that case against the pair based on the judge's planned instructions to the jury.