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The Supreme Court’s decision in the corruption case against former Virginia governor Robert McDonnell will complicate the already complex investigation of union influence in the Walsh administration.

The court found that it was not a crime to receive money from a constituent in exchange for introducing him to officials who might be interested in his business and even hosting him at the governor’s mansion.

What was illegal was taking money in exchange for a decision on a pending matter, like the award of a specific government contract or a vote for bill. The decision was unanimous and followed other Supreme Court decisions rejecting the government’s novel interpretation of federal criminal statutes.

While the charges leveled against McDonnell are different from those against Walsh officials Kenneth Brissette and Timothy Sullivan, the court’s message was unmistakable: Criminal law is not infinitely malleable. You may not like the politician’s behavior – Chief Justice John Roberts called McDonnell’s conduct “tawdry” – but unless the law clearly makes it criminal, you cannot prosecute him for it. (Disclosure: I, along with a number of other law professors, signed an amicus brief urging the position the court took in the case.)

In fact, to Roberts, being responsive to constituents was part and parcel of American democracy: “The basic compact underlying representative government assumes that public officials will hear from their constituents and act appropriately on their concerns,” even constituents who had made campaign contributions. (This is a theme reflected in the Citizen’s United case; campaign money for “access and ingratiation” was just fine). One of the examples of lawful conduct Roberts gave involved union advocacy – a meeting with a union official worried about a plant closing.

Like it or not, unions are among Boston’s most important constituents. Union advocacy is baked into municipal government; unions have formal representation – literally a seat at the table – in a number of Boston boards and commissions. By statute, for example, the Zoning Board of Appeals reserves places for union members, and, indeed, specific unions.

So what are the lines between appropriate advocacy – constituent services – and illegal conduct? The devil is the details, and right now, apart from leaks and speculation, the indictment is the only official document there is. It alleges that Brissette and Sullivan advocated on behalf of the unions – telling one company that filming had to be “in a union environment” – but also that they directly interfered with the process of getting permits. And it suggests that all this began with the Walsh administration, pointing out (in a statement more political than legal) that “in January 2014, the administration of the City of Boston changed.”

The history of this city is filled with examples of statements by powerful municipal officials encouraging – even directing – union participation in companies doing business here. In May of 2007, for example, the City Council passed a resolution urging ClearChannel to resume union negotiations. Mayor Tom Menino followed up with an open letter to the company decrying the use of replacement workers.

Still, the claims that Brissette told a location scout to hold a permit because the film company was not union or that both men pressured another company to hire members of a particular local are troubling, steps seemingly beyond appropriate advocacy. But wrongheaded – even tawdry conduct, to use the Supreme Court’s word – does not necessarily establish a federal criminal violation. Neither man had the authority to do anything with respect to the permits (unlike McDonnell, whose introductions to the right people carried considerable weight). Nor is there any allegation that either got anything in return for his acts, except the enduring support of an important constituency. Extortion typically involves a “quid pro quo,” getting money or something of value in exchange for a specific official act.

The Brissette/Sullivan charges are novel, to say the least. However much the public is rightly concerned about their alleged conduct, the federal criminal law is not supposed to be stretched to cover every kind of misbehavior by state public officials. That, at its core, was the court’s message in McDonnell.