– No one knows how special counsel Robert Mueller’s investigation into Russian political interference and potential White House obstruction will end, but Mueller is already changing how the nation’s capital does business.

His prosecutors have taken the rare step of pursuing some of President Donald Trump’s former senior aides for not registering as lobbyists for foreign governments, rattling the rarefied world of highly paid professionals who advocate in Washington for traditional foreign allies, unsavory strongmen and other overseas clients.

Partly as a result, the number of people registering as “foreign agents” for new clients — meaning they lobby for foreign interests — jumped from 68 in 2016 to 102 in 2017. A total of 422 such lobbyists are currently registered, although some lawmakers believe many more are still in the shadows.

“Any firm that’s worth their salt is going to be doing some due diligence,” said Thomas Spulak, a partner at the King & Spalding law firm who helps clients comply with the Foreign Agents Registration Act, known as FARA. “And if not, they have their head in the sand.”

Scrutiny from the small Justice Department unit that enforces the law has ebbed and flowed over the years, lawyers said, depending on what scandals are making headlines.

“Then it comes out with a vengeance,” said Grayson Yeargin, a partner at the Jones Day law firm. “And right now, it’s probably more active than it has been.”

Lawmakers are considering new steps to beef up the 1938 law, and lobbyists are reading the fine print to make sure they aren’t in violation — a crime that could lead to five years in prison.

“There’s just no point in taking a chance on this stuff,” said one lobbyist who requested anonymity because he’s in the middle of updating disclosure forms to ensure compliance.

Lawyers said lobbyists generally had steered clear of brazenly flouting FARA, which was originally passed to flush out Nazi agents before World War II. In recent years, the law wasn’t a major source of concern on K Street, the traditional home of the capital’s top lobbying shops — sort of like driving over the speed limit because no cops are in sight.

Then came the indictment last October of Paul Manafort and Rick Gates, two high-powered business partners and political consultants who had served as chairman and deputy chairman, respectively, of Donald Trump’s 2016 presidential campaign.

Among the dozen charges they faced was one for not properly disclosing their extensive — and highly lucrative — work for the Kremlin-backed government in Ukraine. In all, prosecutors said, they moved more than $75 million through offshore accounts.

Gates pleaded guilty to two separate crimes last month in return for cooperating with the Mueller probe, and the other counts were dropped. But Manafort still faces the FARA charge plus nearly two dozen other criminal counts in Washington and Virginia. He has pleaded not guilty and the first trial is scheduled for Sept. 17.

Mueller also used the foreign lobbying law to pressure Michael Flynn, Trump’s former national security adviser.

In his plea agreement, Flynn admitted to lobbying for Turkey’s increasingly authoritarian government while working for Trump’s campaign, but without registering as a foreign agent. He wasn’t charged with violating FARA, however, and he pleaded guilty to a single count of lying to investigators about his conversations with a Russian diplomat.