The trial of two Boston city hall officials is getting underway this week. Timothy Sullivan and Kenneth Brissette, both senior appointees of Mayor Marty Walsh, have been charged in federal court with extortion. The men are accused of illegally pressuring the Boston Calling music festival in 2014 into hiring union labor. The case against the two men has been in limbo for three years, but now it is moving ahead, with opening arguments set for Tuesday. As the trial proceeds, Walsh could be called as a witness. WGBH News' Isaiah Thompson has been keeping track of things. This transcript has been edited for clarity.

Arun Rath: It’s been a while, so I think a lot of us would benefit from what exactly federal prosecutors are accusing Kenneth Brissette and Timothy Sullivan of doing.

Isaiah Thompson: The indictments came down in 2016, and federal prosecutors are charging the two men with extorting the Boston Calling music festival into hiring laborers from the stage hands union that the festival said it didn’t want and didn’t need. They’re accused specifically of using their position and their power as senior members of the Walsh Administration to exert pressure on the festival to hire these workers that they didn’t want to hire.

Rath: How did they do that, allegedly, in terms of putting the pressure on?

Thompson: According to the indictment, there were a series of events as the festival was preparing to open. Brissette and Sullivan, working together allegedly, were meeting with Boston Calling folks, corresponding with them, and exerting this constant and increasing pressure, saying 'Hey, you need permits, you need this, you need that, you need to hire union labor.' And everything sort of culminated, according to the indictment, about three days before Boston Calling was supposed to open, where Brisette allegedly said, 'Hey, you need to hire folks now.' And in the end, Boston Calling did hire a number of workers from the stage hands union.

Rath: Why has the case dragged on so long? The men were charged three years ago.

Thompson: There’s been a very contentious legal issue at the center of this case which revolves around the definition of extortion. Lawyers for the defendants have strongly argued that even if you took all the facts being laid out, that’s not extortion. These men were advocating for union labor, they were working for a mayor who is a proudly pro-union mayor, and the defense has argued consistently that this is not extortion — this is lawful advocacy on behalf of unions. And in this legal back and forth, there was a real question as to whether the definition of extortion really applied to this case, and at one point last year, prosecutors admitted that they couldn’t prove that either Brissette or Sullivan had sought to gain anything personally. Defense lawyers said that’s not extortion, and the judge agreed with that line of reasoning, broadly, and threw the case out. However, a few months ago, an appeals court reversed that decision and now this is going forward, finally. The question will be whether a jury agrees that these men were involved in a racketeering extortion enterprise, or whether they were trying to get a better deal for unions in a union city.

Rath: I can’t let you go without asking about the political impact. These were aides to Mayor Marty Walsh. What is at stake potentially for the mayor?

Thompson: It’s important to note that the mayor has not been accused of any wrongdoing at all. However, these were two senior members of his staff. And part of the allegation here is that the reason these two men were using their power and their influence to put so much pressure on Boston Calling to hire union labor was that in a pro-union administration, it would benefit them, it would advance them, and it would please their boss, Marty Walsh. Now, no one has alleged that Marty Walsh had any knowledge of this or anything to do with it, but in trials, inevitably a lot of details come out, and I think that possibly the most worrying thing for the Walsh Administration right now is simply not knowing who is going to say what up on the stand.

Rath: Thanks for getting us up to speed, Isaiah.

Thompson: Thanks for having me.

Rath: That’s WGBH Radio’s Isaiah Thompson, giving us an overview of the Boston Calling case, which is getting underway in federal court. This is WGBH’s All Things Considered.