With the jury hopelessly deadlocked, a federal judge was forced to declare a mistrial in the bribery trial of US Sen. Bob Menendez (D-NJ). Who can blame the jury? Its members had been asked to answer some of the hardest questions in modern anti-corruption law.

Is it criminal bribery when a $600,000 donor to a Super PAC asks for, and gets, help pushing a policy change that benefits him — from the candidate the Super PAC supported?

How explicit do agreements to trade favors have to be for a charge of criminal bribery to stick? How do we tell the difference between a gift as a sign of friendship and a gift as a bribe? Does it matter that everyone else is doing the same thing?

Sen. Menendez’s trial — his co-defendant was the eye doctor Salomon Melgen — was fascinating not because of any scandalous revelations, but because of the lack thereof: There were no smoking-gun letters in which Menendez offers a governmental favor for rolls of cash, or wiretaps on which he is overheard offering a seat on a board in exchange for getting Melgen’s son a job. And despite what you might think reading the right-wing Twittersphere, he was not charged with anything connected to prostitutes, period. (The judge refused to allow any mention of unsupported allegations that Menendez solicited prostitutes during a Caribbean vacation.)

Instead, what was fascinating is that Menendez’s behavior is right on the razor’s edge between legal and illegal, at a time when the definition of “legal” is changing.

I’m with the jury: Even after closely following the trial, I have no strong view on Menendez’s guilt or innocence, given the laws they have to work with. I do have a view, however, that the Supreme Court has been playing a shell game with corruption laws. It has stripped anti-corruption legislation of its power in two areas: campaign finance laws and anti-bribery laws. The public is left with little recourse against a growing threat of corruption. Whatever happens with this particular case, this is no way to do corruption law.

The essence of the charges against Menendez and the ophthalmologist Melgen boil down to this: Melgen gave Menendez a combination of personal favors, including flights and vacations, and political favors, including campaign contributions and those Super PAC contributions. In exchange, Menendez used the power of his office to serve Melgen in three different areas: He pushed for a Medicaid reimbursement policy change, he intervened in a dispute about enforcing a company’s contract for port security with the Dominican Republic, and he helped Melgen’s friends get visas.

Melgen cared deeply about the Medicaid reimbursement because he was charged with overbilling under the policy, and he owned a 50 percent stake in the company involved in the port security dispute.

There is also a charge that Menendez violated the false statements law — the law, incidentally, that the former Trump policy adviser George Papadopoulos recently pleaded guilty to — when he signed his annual financial disclosure forms, filed with the Senate. He did not include gifts from Melgen.

The defense admits the ophthalmologist provided those flights and vacations, but says they were gifts from a friend and not part of any quid pro quo. They admit to the campaign and Super PAC contributions (hardly a secret in any case), but say they simply signify political support. The defense similarly admits Menendez pushed hard for the Medicaid and port security policy changes, but say Menendez believed in the policies he was pushing. He was influenced by Melgen as friend, not as a donor.

A revolution in anti-corruption law

The case came at a time when the law of corruption is undergoing a massive shift. (Federal prosecutors must now decide whether they want another trial.) That shift is the result of changing Supreme Court interpretations of the two main kinds of anti-corruption laws.

The first kind is prophylactic laws — those intended to make corruption less likely. These laws are like speed limits. We outlaw people driving at 90 mph not because all, or even most, of those drivers will kill someone. We do not think there is a deep moral difference between driving 65 or 72 mph on a clear, dry, sunny day on an empty highway. Rather, we think that a simple, clear ban on certain behavior will make death and injury far less likely, in general.

In the anti-corruption realm, such laws include campaign finance laws that create absolute limits how much corporations, labor unions, and foreign governments can spend. Such laws prohibit behavior not because it is per se corrupt, but because it is likely to lead to corruption.

For instance, foreign governments cannot donate something of value to candidates for office, period. We don’t care if there’s evidence that the candidate agreed to give the foreign government a favor; we care that there’s a risk. The advantage of prophylactic laws is that we need not engage in deep fact-finding about intentions, peering into the heart of a donor or senator who violates them. We simply ban the interaction.

The Supreme Court has been whittling away at corruption prevention laws

Over the past decade, starting with a 2007 case, Wisconsin Right to Life v. FEC, and most prominently including Citizens United, the Roberts Court has been at war with these prophylactic laws — concluding that they are over-inclusive and that they chill free speech. In Wisconsin Right to Life, the Supreme Court struck down a section of the McCain-Feingold Act that limited the ability of corporations and unions to run TV ads. The provision said they could not fund broadcast ads costing more than $10,000 that mentioned a candidate for federal political office within 30 days of a primary election or 60 days of a general election.

It was a classic prophylactic rule, one with clear boundaries. Think of it as a speed limit.

Justice Roberts, writing for the Court, said the rule infringed on free speech rights and didn’t precisely target corruption — because, he said, corruption only means quid pro quo deals, and there was no evidence of massive quid pro quos between corporations and candidates.

Three years later, in Citizens United, the Court struck down yet another powerful prophylactic rule. The law had been that corporations and unions can’t spend unlimited money that directly advocated the election or defeat of a candidate. The Court again said that this law violated the First Amendment (because citizens have a right to hear corporate and union political speech). The Court said — again — that it didn’t target quid pro quo corruption precisely enough, because there was insufficient evidence that corporations and unions were engaged in explicit deals with candidates.

Citizens United opened the floodgates and allowed the creation of Super PACS — entities that take and spend unlimited corporate, union, and individual money and advocate explicitly for the defeat or election of individual candidates.

The Menendez trial was a direct outgrowth of Citizens United. The activity that was banned before Citizens United was at the heart of one of the charges against Menendez. The hundreds of thousands of dollars donated to a Super PAC was the alleged quid for some of Menendez’s actions. Without Citizens United, this situation could not have occurred — nor a charge resulted — because the campaign vehicle of the Super PAC had been prohibited as a prophylactic.

Bribery laws have also been hollowed out

Here’s what should madden you: In his decision in Citizens United, Justice Kennedy said that we should not worry corporate spending would lead to greater corruption, because bribery laws would still deal with our corruption problems. Then the Court went ahead and weakened bribery laws.

These laws include the Hobbs Act, mail and wire fraud, the Travel Act, and the federal bribery and gratuities statute. The Court has been extremely skeptical of juries’ decisions where such laws are concerned, has expressed the concern that enforcement of these laws chill normal political behavior, and has been very worried about prosecutorial overreach.

When a trade association was convicted of illegal gratuities after giving Secretary of Agriculture Mike Espy tickets to the US Open tennis tournament, expensive luggage, and other gifts, the Court overturned the conviction — and preexisting law — because of an absence of a clear connection between the tickets and Espy’s favorable treatment of the trade association.

Then in last year’s McDonnell decision, the Court overturned a jury verdict that found Virginia Gov. Robert McDonnell guilty of bribery — and in doing so narrowed the scope of federal bribery laws.

The basic structure of most bribery laws is that they prohibit a) giving/receiving a thing of value b) in exchange for c) a governmental action. McDonnell took tens of thousands of dollars in gifts, including a Rolex watch; then he used his office to make calls to help his benefactor. There was no problem with the evidence that McDonnell took something of value, or that it he did it in exchange for something. However, the Court held that the jury had not been correctly instructed by the judge about the standards for what constitutes an “official act” — the pro in a quid pro quo.

An official act, the Court said, cannot be merely “arranging a meeting, contacting another public official, or hosting an event” without something “more.” Instead, the Court said, the government could only bring a case where there was evidence that the governor took the Rolex in exchange for McDonnell formally exercising governmental power, or attempting to do so.

McDonnell led to high-profile convictions being overturned — including those of former leaders of the New York state legislature Dean Skelos (R) and Sheldon Silver (D), and former Congress member William Jefferson (D-LA), who was caught with $90,000 in his freezer — most which will now be retried with new jury instructions. But it has also led to many other questions about the status laws that seek to punish bribery after it has occurred.

For instance, the defense lawyers in Menendez’s case argued strenuously that a longstanding body of bribery law, “stream of benefits” bribery, was wiped out by McDonnell. “Stream of benefits” has been recognized by courts where the private party gives something of value to a public official, who then essentially agrees to help out as opportunities arise. The particular help isn’t specified beforehand. The illegal agreement is an exchange between a gift and a promise to help in the future.

According to the defense team, this can’t be reconciled with McDonnell. The judge in the Menendez trial (rightly in my view) rejected the defense’s arguments, leaving for another day (and perhaps appeal) a full discussion of how the longstanding “stream of benefits” bribery squares with McDonnell.

In the Menendez case, there was proof that the senator sought specific governmental actions (changing Medicaid reimbursement policy, for instance), so the particulars of McDonnell are not really at issue here. But the specter of McDonnell is here nonetheless: The case was tried in the shadow of a Supreme Court that repeatedly chipped away at criminal bribery laws, and seems overly concerned about protecting lawmakers, especially when engaged in activities that have become routine.

Finally, the Menendez trial comes smack in the middle of a 25-year confusion about the correct jury instructions in cases in which the quid, the thing of value, is a political donation. The Supreme Court has said that more explicit promises are required when campaign contributions are involved, but has not been clear about just how explicit. The need to clarify this area of the law has taken on new urgency since the creation of Super PACs, especially because increasing numbers of Super PAC donors are getting what they want.

It is fitting that the trial ended with a hung jury. The Court has struck down so many laws that would have made this case easier. If laws prohibiting Super PACs were still in place, we’d have no $600,000 donation. But in the very case enabling Super PACs, Citizens United, the Court suggested that bribery laws would be powerful tools to combat corruption threats — and then went ahead and weakened those laws.

Whether or not you think Menendez is guilty, you may not want your senators getting the benefit of hundreds of thousands of dollars in campaign contributions and then helping those contributors. And while we cannot peer into Menendez's heart, we know that other politicians have certainly befriended wealthy individuals who then go on to make Super PAC contributions.

Was it friendship? Was it corrupt? Or was it our fault for creating a system that encourages “friendships” that blur the line?

Zephyr Teachout is an associate law professor at Fordham Law School and author of Corruption in America: From Benjamin Franklin's Snuffbox to Citizens United (Harvard 2014). Find her on Twitter @ZephyrTeachout.

The Big Idea is Vox’s home for smart discussion of the most important issues and ideas in politics, science, and culture — typically by outside contributors. If you have an idea for a piece, pitch us at thebigidea@vox.com.