Senate Judiciary Chairman Chuck Grassley on Monday welcomed the Justice Department's decision to enforce the Foreign Agents Registration Act against two former Trump campaign advisers but warned that the law should be enforced equally to members of both parties.

“I’ve been raising concerns about lackluster enforcement of this foreign influence disclosure law for years now, regardless of administration or political party,” Grassley said in a release. “It should be enforced fairly and consistently, regardless of politics or any other factor.”

Former Trump campaign manager Paul Manafort and his former business associate Rick Gates were charged with failing to disclose their work for foreign agents, among other charges that included money laundering and conspiracy against the United States. Both men turned themselves in to federal authorities early Monday morning.

FARA requires anyone who is doing political work on behalf of another country or working on behalf of politicians or agents of another country, to disclose their interests to the Department of Justice so the public can see who is trying to influence U.S. policy and public opinion.

Since 2015, the Iowa Republican has been raising questions about a lack of enforcement of the law by the Justice Department after news reports appeared to show Hillary Clinton advisor Sidney Blumenthal was operating as an “off the books” intelligence resource for certain foreign interests and the State Department.

Manafort and his consulting firm are known to have worked on behalf of some political figures associated with Ukraine, and he belatedly filed his FARA disclosure this summer.

“The dirty little secret is that lots of people across the political spectrum in Washington have skirted their FARA registration obligations for years with little to no accountability,” Grassley said. “I’ve been working on legislation to improve the Justice Department’s enforcement of FARA, and expect to introduce it very soon.”

While FARA has gained greater interest because of Russia’s attempts to influence the 2016 elections, very few prosecutions have been sought using the law. Only one case has been successfully tried in court in the last 40 years, and only a small handful of other cases were filed, some of which resulted in plea bargains.

Besides Grassley’s interest in the Blumenthal matter, he’s also raised FARA issues regarding Trump’s former national security advisor Mike Flynn, and also with Fusion GPS. Some have accused Fusion of working on behalf of Russian interests to overturn the Magnitsky Act, which freezes the assets of Russian politicians and oligarchs which are held in Western banks.

In a July hearing on FARA enforcement, Grassley bemoaned a seeming lack of interest by the Justice Department in FARA and noted that the division that enforces the law had sent 178 letters to people they believed were in violation of the law, over a little more than a decade.

“That’s only 15 letters a year," Grassley said. "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.”