The release Thursday of special counsel Robert S. Mueller III’s report raised urgent questions about obstruction of justice, foreign influence on U.S. elections and what will happen next, now that the investigation is finished.

But Mueller’s work may leave another legacy that has nothing to do with President Trump’s 2016 campaign. The special counsel’s investigation has also revived a criminal law that had gone unenforced for decades — one that could, if authorities begin taking it seriously again, change the way business is done in Washington’s establishment.

The Justice Department, in connection with Mueller’s work, brought charges against seven people under the Foreign Agents Registration Act (FARA). The statute is designed to give the American public detailed, meaningful information about how foreign powers use lobbyists and public-relations firms to influence U.S. trade and military policy. FARA requires foreign agents — people paid by foreign governments or foreign political parties, among others — to register, file contract details, and keep copies of all contracts and correspondence.

Among others, Mueller indicted Trump’s campaign chairman, Paul Manafort, and former national security adviser, Michael Flynn, on charges involving FARA filings, as well as Manafort’s business partner, Rick Gates. On April 11, former Obama White House counsel Gregory Craig was charged with lying about FARA in connection with work he did with Manafort in Ukraine.

The seven FARA cases brought in connection with Mueller’s investigation equaled the total number of FARA charges filed by federal prosecutors between 1966 and 2017.

Calling the law rarely used, as many news stories on Mueller’s charges have done, is the understatement of the decade. A long-standing public Justice Department handbook casually noted that FARA “is, after all, a malum prohibitum, little known outside the legal community.” The manual explicitly stated that voluntary compliance was the goal. Failing to register as a foreign agent or fully comply with FARA is punishable by serious prison time. It is also so routine that Ken Silverstein, a journalist who has reported extensively on foreign influence-peddling, believes that “roughly half of Washington would be under arrest” were it fully enforced.

When FARA works as intended, it provides the public a glimpse of one of the ugliest practices in lobbying: pay-to-play diplomacy, in which foreign governments pay law and lobbying firms to make campaign contributions to candidates to shape policy.

Because of FARA filings, we learned that in the days before a key vote on a congressional resolution to end U.S. involvement in the war in Yemen, lawyers working for firms paid by the Saudis and the United Arab Emirates gave more than $200,000 in campaign contributions to key lawmakers. As we like to say in law: Res ipsa loquitur. We know that a firm that was paid almost half a million dollars by the Saudi government in 2017 made a $2,000 campaign contribution to a critical senator on the same day the firm talked to his legislative director. We know that Saudi-employed foreign agents donated $400,000 to campaigns in 2017. And we know that Saudi and Emirati lobbyists contacted Senate offices more than 100 times between the introduction of the Yemen war resolution and the vote in March.

A foreign government cannot contribute to a candidate for federal office. However, foreign governments can pay a lawyer to help with access to governmental officials, and pushing client business with the access purchased through campaign contributions is implicitly endorsed in Citizens United v. Federal Election Commission , the 2010 Supreme Court ruling on campaign expenditures by corporations. As Justice Anthony M. Kennedy wrote, “A substantial and legitimate reason, if not the only reason, to cast a vote for, or to make a contribution to, one candidate over another is that the candidate will respond by producing those political outcomes the supporter favors.”

Foreign governments effectively launder foreign cash, directed at members of Congress, through white-shoe law firms. Of course, they don’t literally do that: Instead, they handsomely pay lawyers or lobbyists, who then individually make contributions to campaigns, knowing that those contributions are vital to maintaining access and influence.

Because the moral logic of many elite lawyers can be summed up by three factors — it isn’t illegal, it isn’t tobacco and other people do it — this practice happens all the time. The only real tool to expose this legal foreign money laundering is FARA, and for years, the law has been treated as something like a joke.

In the past two years, FARA filings documented an average of $37 million in mostly governmental money spent each month, with 1,302 lobbyists registering since 2017. But we have no idea how many people do not file. We know that relying on voluntary compliance is ineffective: A 2016 Justice Department audit found that between 2013 and 2015, 62 percent of new registrants filed documentation late, and 47 percent of informational materials failed to include the required disclosure statement. These numbers tell you just how poorly the system is working. When so many registrants brazenly fail to follow all aspects of the law, it is a good bet that many more don’t even bother with the filing.

But Mueller has single-handedly changed the climate around FARA. On April 12, W. Samuel Patten, a political consultant, pleaded guilty to failing to register as a foreign agent for a Ukrainian political party aligned with Russia. The day before, the Justice Department announced the indictment of Craig, who has pleaded not guilty.

Craig — a product of Yale University, the late senator Edward Kennedy and the Obama White House — is arguably at the apex of Washington’s retainer class. The government alleges that he concealed information and made false statements to the Justice Department’s FARA unit. I take no position on Craig’s guilt or innocence in this case, but for someone of his standing to be charged in anything FARA-adjacent is astonishing in Washington.

Some people may see this spate of cases as a one-off, but now that prosecutors have rediscovered this powerful tool, they aren’t likely to forget about it again. At a minimum, they may start using FARA violations as leverage in other investigations.

My hope is that it won’t just be used as a bargaining chip, however, but treated as a serious law, protecting Americans from secret foreign influence. As the judge in the Patten case pointed out, FARA filings are not a mere technicality but essential to democracy.

In other areas, lobbyists defend their work as essential in educating lawmakers. We need lobbying, they say, so members of Congress can learn in textured detail about what is happening outside Washington. This argument is easily enough abused, but the premise is patently false when it comes to foreign governments: Whole embassies are staffed with diplomats ready to provide information about the needs of Ukraine, Saudi Arabia, Ireland, Israel or China.

Foreign governments using American law firms to push policy transforms the diplomatic relationship. The privatization of diplomacy through lobbying undermines an imperfect but powerful system and replaces it with pay-to-play. Foreign governments don’t rely exclusively on staff diplomats, because they know adding money to the relationship works. Foreign policy is particularly prone to legal corruption because it falls in a sweet spot where three factors converge: low public information, few opponents and enormous payoff for foreign powers.

So the real punch in FARA — and where the public stands to gain — lies in the law’s requirement that lobbyists include copies of each written contract and a full, detailed statement of the political activity for which they are engaged. A law firm paid by the Turkish government can’t just mention a contract; it must also include whether the contract is designed to cover up the Armenian genocide or promote learning the Turkish language in schools.

FARA, if taken seriously, could become a fundamental protection for the integrity of our trade and military choices. Think of it like campaign finance reporting laws or the mandatory reporting to the Securities and Exchange Commission. Transparency may not change all the incentives, but it can give us a clear window into what is happening — and might lead to change.

At the Constitutional Convention, our framers worked obsessively to protect against corruption, particularly foreign countries that would seek out structural weaknesses in our country to corrupt our politics. In various debates, New York delegate Alexander Hamilton imagined “foreign powers” that would “interpose,” and Massachusetts delegate Elbridge Gerry predicted that “persons having foreign attachments will be sent among us & insinuated into our councils, in order to be made instruments for their purposes.”

FARA can be a powerful tool for detecting those foreign instruments. We should use it. No matter whom it ensnares.

This article has been updated since it was originally published on April 17.

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