Former Trump campaign manager Paul Manafort and his associate, Rick Gates, are facing charges that they failed to register as agents of a foreign government, a rare charge that has only led to one successful conviction in the last 40 years.

The charges essentially mean Manafort failed to file a disclosure with the Department of Justice that acknowledged his work for a foreign nation or foreign politicians as required by the Foreign Agents Registration Act (FARA), which became law in 1938.

Manafort did work for political figures from the Ukraine in previous years, and his consulting firm did register retroactively with FARA just a few months ago.

According to a 2016 audit of FARA enforcement by the DOJ’s inspector general, “[b]etween 1966 and 2015 the Department only brought seven criminal cases – one resulted in a conviction at trial for conspiracy to violate FARA and other statutes, two pleaded guilty to violating FARA, two others pleaded guilty to non-FARA charges, and the remaining two cases were dismissed.”

The same report noted that the Justice Department over the years has more often sought to prod violators into filing retroactively rather than pursuing criminal charges.

“The basic idea is transparency,” Senate Judiciary Chairman Chuck Grassley said at a committee hearing on FARA in July. “Foreign propaganda and lobbying should not be underground, but out in the open. In general, FARA doesn’t make these activities illegal. It just says you have to register with the Justice Department. FARA is a public disclosure law. The government, and the people, need to know if foreign governments, political parties, or other foreign interests are trying to influence U.S. policy or public opinion.”

Grassley noted that the Justice Department has sent about 178 letters over a period of 12 years to people that it believed needed to register as a foreign agent.

“Only 178 letters in 12 years,” Grassley said. “That’s only 15 letters a year. The Justice Department thinks I send more oversight letters than that in a week. Only nine people in the entire Justice Department work full time to enforce this law and monitor potential unregistered foreign agents in the U.S. It’s no surprise that only 400 foreign agents are currently registered.”

The inspector general audit also noted that the Justice Department has “not sought civil injunctive relief under FARA since 1991.”

Nonetheless, it's use against Manafort could revive claims from Republicans that Democrats are also at risk of violating the law. Conservatives and Republicans have long argued that some of Sidney Blumenthal’s work in 2015 as an informal go-between for foreign nationals and then-Secretary of State Hillary Clinton may have violated FARA laws.

“Mr. Blumenthal repeatedly sent Secretary Clinton intelligence memos and at one point Secretary Clinton told him to ‘keep them coming,’” Grassley said in the July committee meeting. “News articles reported that Mr. Blumenthal transmitted documentation to Secretary Clinton on behalf of the Georgian Dream, a political party in the country of Georgia.”

“Now, if the Justice Department thought Mr. Blumenthal’s activities on behalf of foreign interests did not require registration under the law, it should say so,” Grassley concluded. “After all, if he did the right thing by not registering, then to be fair to him, the Department should explain why.”

FARA registration may also be an issue for Fusion GPS, the company that put together the "Russian dossier" on President Trump.

Some have accused Fusion GPS of orchestrating a campaign within the U.S. on Russia's behalf to have the Magnitsky Act overturned, a law that the Kremlin is known to oppose vehemently because it freezes the assets held in Western banks of some Russian officials.

Bill Browder, an international investor and financier who urged U.S. Congress to pass the law, told senators in July that he believes Russia was behind that group's effort, but he said in August he was not aware of any effort by the DOJ to investigate.