Seven Senate Democrats want the Justice Department to review whether Rudy Giuliani is complying with the law mandating that American advocates must register when they're working on behalf of foreign clients.

Go right ahead, Giuliani said.

"Let them knock themselves out," the former New York mayor told NPR.

The senators — Tom Udall, Elizabeth Warren, Tammy Duckworth, Richard Blumenthal, Sheldon Whitehouse, Jeff Merkley and Dick Durbin — pointed to reports they said suggest Giuliani is not only working for President Trump, but representing foreign clients without disclosing that as required by the Foreign Agents Registration Act.

"We write regarding public reports that suggest the president's personal attorney ... has conducted a number of political and public relations activities on behalf of foreign entities," they wrote on Thursday to John Demers, the assistant attorney general who leads the National Security Division of the Justice Department.

Continued the senators: "Mr. Giuliani's numerous foreign clients and ongoing communications with senior U.S. government officials raise significant concerns."

They cited his reported links to the Party of Regions in Ukraine, Iran's Mujahedin-e-Khalq, or MEK, and "unnamed clients in Colombia and Brazil."

Also: "Press releases on Mr. Giuliani's security company's website describe additional work on behalf of the governments of Chile, Dominica, Uruguay, the Dominican Republic and El Salvador," the Democrats said in their announcement.

Giuliani said he has never represented a foreign government in or before the United States.

"I have done work for foreign entities, but in those countries," Giuliani said. "Basically it's law enforcement and security work having nothing to do with or affecting financial interests of the United States of America — which is what FARA covers."

The once-obscure law has taken on new relevance since the appointment of Justice Department special counsel Robert Mueller. Prosecutors have begun using it — including against President Trump's former campaign chairman, Paul Manafort — after years of what was considered lax enforcement.

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