BOSTON — A federal judge on Thursday dismissed charges against two against two Boston mayor's aides accused of bullying music festival organizers into hiring union workers by withholding city permits.

Judge Leo Sorokin's ruling comes days before Kenneth Brissette and Timothy Sullivan's closely watched extortion trial was set to begin in Boston's federal courthouse. Prosecutors, who complained the judge's proposed jury would sink their case, didn't fight the dismissal but indicated they would seek an appeal.

Prosecutors said Brissette and Sullivan told organizers of the 2014 Boston Calling music festival — Crash Line Productions — that if they didn't hire from the union, workers would picket and bring a giant inflatable rat, "which would be a problem for both Crash Line and the mayor."

Crash Line ultimately agreed to hire a handful of union workers and got the necessary permits, court documents say.

Prosecutors disputed the judge's proposed jury instructions on the definition of "obtaining" property — in this case, wages from the union jobs — under anti-extortion law.

The judge said the government would have to show that the men personally benefited from the hiring of the union workers, but prosecutors said that reading was too restrictive and would prevent them from putting on their case.

In court documents, prosecutors suggested the men may have personally benefited because Democratic Mayor Marty Walsh was backed by several unions during his campaign and "some members of his administration assumed that unions would be among his preferred constituents."

Walsh was head of an umbrella group of local construction unions before he took office in 2014.

In his ruling Thursday, the judge called the matter an "unusual case" and said the dispute over the legal standard should be settled first to spare the court and the public the expense of a trial.

A spokeswoman for the U.S. attorney's office declined to comment Thursday. Attorneys for Brissette and Sullivan didn't immediately respond to emails seeking comment.

The mayor said in a statement that he appreciates this "being brought to resolution."

The dismissal is the latest blow for the Massachusetts U.S. attorney's office in a string of cases examining the bounds of lawful organized labor practices.

In August, four teamsters were acquitted in a similar case of using strong-arm tactics to try to extort jobs from a nonunion company filming the reality TV show "Top Chef" in the Boston area. Prosecutors said Brissette also pressured the filming company to hire union workers, but Brissette wasn't charged in that case.

A month later, 1st U.S. Circuit Court of Appeals reversed multiple extortion and racketeering convictions against two Teamsters found guilty in 2014 of threatening to picket businesses unless they hired union workers.

The judges said former Teamsters John Perry and Joseph Burhoe didn't commit extortion because the defendants were seeking real jobs for themselves and union members.