Robert Mueller is breathing new life into a World World II-era law that requires lobbyists hired by foreign governments to register with the Department of Justice and document their efforts.

The special counsel has now brought a half dozen indictments utilizing the law in cases related to Russian efforts to influence the 2016 presidential campaign. And more cases are reported to be in the works.

The most high profile of these cases have involved central members of President Donald Trump’s campaign team, including former chairman Paul Manafort, who was convicted of engaging in a secretive lobbying operation for Kremlin-backed Ukrainian president back in 2012, his longtime associate Rick Gates, and Trump's former national security adviser Lt.Gen. Michael Flynn, who admitted to a foreign lobbying violation when he pleaded guilty to making false statements.

Investigators have reportedly been looking at the same law as they scrutinize Democratic operative Tony Podesta, former Minnesota Republican Rep. Vin Weber and former Obama White House counsel Greg Craig, who were all involved in Manafort's Ukraine-related work.

The succession of high-profile cases has served as a wake-up call for lobbyists and consultants in Washington, according to experts who have argued that compliance had grown lax. There was, they said, a mistaken belief that the Foreign Agents Registration Act (FARA) had wilted with age.

“It was a serious eye-opener,” said Caleb Burns, a lawyer who has expertise in political and lobbying laws.

The registration requirements, commonly referred to as FARA, were crafted in 1938 to expose efforts by Nazi sympathizers to try and influence U.S. government activity leading up to World War II. FARA requires individuals and entities acting as an agent of a foreign client to register under the Department of Justice.

The demands on those who register are substantial -- filings are supposed to include details about money spent and contacts made on behalf of the foreign client, as well as a full accounting of the payments.

But few cases have been brought to prosecution.

According to a Department of Justice Inspector General’s audit report, from 2016, between 1966 and 2015, federal prosecutors brought only seven criminal FARA cases, and only three of them resulted in convictions.

In recent years there has been a gradual decrease in the number of lobbyists filing registrations. Experts say the decline is largely due to the passage of new lobbying rules that let consultants working for foreign clients that were not directly tied to governments file a relatively simple, and less revealing, disclosure form instead.

Josh Rosenstein, a lawyer with expertise in FARA compliance, said the Justice Department typically has followed its formal policy of encouraging voluntary compliance over prosecution. The rare instances in which violations led to prosecution, Rosenstein said, have been high-profile cases involving unfriendly regimes.

"FARA has been dangled as both a carrot and a stick as part of a negotiation for a plea," Rosenstein said.

For the special counsel's office, it has been used a stick.

Manafort, Gates and Flynn retroactively registered as agents under FARA once it became clear they were under investigation, but the special counsel was not satisfied by that in all three cases.

The allegations have helped the special counsel team's gain leverage as they tried to flip witnesses. On their own, violations carry a maximum fine of $10,000 and five years in prison.

The Mueller team’s tactic of locking down cooperating witnesses using FARA violations follows a standard prosecutorial tactic, said Daniel Petalas, a former federal prosecutor who has expertise in election and lobbying laws. The Mueller team’s actions came as the Justice Department was initiating its own crackdown on FARA, Petalas said.

"FARA charges have constituted a key part of the mosaic of [the special counsel's] approach to prosecuting many of these cases," said David Laufman, the former chief of counterintelligence and expert control at the Department of Justice.

Gates in February 2018 pleaded guilty to reduced charges and agreed to cooperate with Mueller’s investigators. Manafort in September also pleaded guilty and reached a cooperation agreement with the special counsel, but only after going through a weeks-long trial in Virginia with his former business partner Gates as the government’s witness against him.

The high profile use of FARA appears to have been noticed. New registrations are up substantially. In 2017, there were 101 new companies registered under FARA as working for a foreign client and 713 individuals registered as foreign lobbyists, compared to 69 new firms and 549 new lobbyists in 2016. New registrations have only increased in 2018.

“We’ve had an extraordinary uptick in FARA-related work since 2017,” Burns said. “And I suspect many other firms in Washington have seen the same.”

More attention to the law will follow if the Department of Justice follows through on cases against three more high-powered Washington figures who have reportedly been subject to scrutiny for their FARA filings -- Podesta, Weber and Craig. Those investigations are being handled by the U.S. Attorney's Office in the Southern District of New York.

Podesta stepped down from his firm immediately after Manafort and Gates were indicted, and revealed he has been fully cooperating with the special counsel's office. His firm, the Podesta Group, closed in May 2018, after months-long financial struggle. Craig left his firm Skadden Arps amid the controversy. Weber's firm, Mercury Public Affairs, appears to remain strong, boasting new clients on FARA filings almost every month.

Craig's attorney William Taylor and Skadden Arps spokesperson declined ABC News' request for comment. Mercury Public Affairs partner Mike KcKeon was not available for comment by the time of publication. Podesta's spokeswoman Molly Levinson declined to comment.

Sources close to Podesta told ABC News in May of 2018 that the inquiries had stopped earlier in the year. Mercury spokesperson in September 2018 told CNN in a statement, "For more than a year now, we have fully cooperated with the (special counsel)" and that "We will continue to fully cooperated with the government if request."

Prosecutors have also brought charges of FARA violations against two of Flynn’s former business partners, who were involved work on behalf of the Turkish government. Bijan Kian, one of the defendants in the case, pleaded not guilty. The other partner lives abroad and has not yet been arrested.

Laufman, who now advises companies and individuals on FARA, said he believes the increased attention has given an old law new teeth.

"I think it's quite likely that the department will continue to keep its foot on the gas pedal of FARA enforcement in appropriate cases," Laufman said.