Washington — The Senate's top Democrat is calling on the Justice Department to investigate whether Richard Grenell, the acting director of national intelligence, failed to inform the department about work he did for foreign entities before joining the Trump administration, possibly in violation of federal law.

Senate Minority Leader Chuck Schumer sent a letter Tuesday to John Demers, assistant attorney general for national security, asking his office to "immediately investigate" reports of Grenell's work in support of the Hungarian government and a Moldovan politician.

Failure to disclose that work as a foreign agent to the Justice Department could be a violation of the Foreign Agents Registration Act, which requires those who lobby in the U.S. on behalf of foreign entities to disclose their efforts. A search of FARA filings shows Grenell did not register.

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"If the reports regarding the nature of Mr. Grenell's undisclosed work with foreign entities are accurate, he may be subject to potential civil and criminal liability as well as vulnerable to blackmail in his new position in the Intelligence Community," Schumer wrote in his letter.

The Justice Department declined to comment.

President Trump selected Grenell, the current U.S. ambassador to Germany, to replace Joseph Maguire as the acting director of national intelligence last week. A vocal ally of Mr. Trump's, Grenell is expected to serve in a temporary capacity until the president names a permanent director to lead the office that oversees the nation's 17 intelligence agencies.

Grenell has come under scrutiny in the wake of his appointment for consulting work he did before he was confirmed as the U.S. ambassador to Germany in 2018. ProPublica reported last week that Grenell wrote op-eds in 2016 defending Vladimir Plahotniuc, a Moldovan politician, and allegedly failed to disclose payments for his work on behalf of the oligarch.

Plahotniuc was barred by the State Department from entering the U.S. last month "due to his involvement in significant corruption."

Additionally, the Washington Post reported Monday that in 2016, Grenell's public relations firm did media outreach for a U.S. foundation funded primarily by the Hungarian government. The Quincy Institute, a think tank focused on U.S. foreign policy, also reported Monday on Grenell's public relations work for the nonprofit.

Craig Engle, an attorney representing Grenell, told ProPublica and the Washington Post separately that in the instances involving the Moldovan politician and nonprofit tied to the Hungarian government, he was not required to register under FARA.

During his confirmation process to serve in Berlin, Grenell was asked by Democratic Senator Ben Cardin of Maryland about his op-eds on Moldova and whether he was paid for his writings.

Grenell said at the time he did not receive any compensation for his work and believes "strongly in confronting threats to democracy, and all of those views were my own opinion."

He also denied having written the op-eds on Moldova at anyone's request or direction.

"My motivation in writing or speaking on any particular subject is because I think it's important," Grenell said.

Schumer told Demers in his letter "Grenell may have misled Congress during his confirmation hearing."

The New York Democrat said it's "imperative" for the Justice Department to examine whether Grenell broke the law. If he did, Schumer added, Grenell should be referred for enforcement action.

"Any illegal activity would obviously disqualify him from serving as the director of national intelligence or in any position of public trust," Schumer said.

A congressional source told CBS News that Grenell did not mention the work on his SF-86, a form used by the federal government in conducting background investigations, during his confirmation process for his role as U.S. ambassador to Germany. Had he registered under FARA, he would have been required to disclose it.

Senate Republicans appeared to hold back in the immediate wake of Mr. Trump's selection of Grenell to serve as acting director of national intelligence. Senate Intelligence Committee Chairman Richard Burr, a Republican from North Carolina, remained mum on Grenell's appointment until Tuesday, when he told CNN he supported the pick. Senate Majority Leader Mitch McConnell did not mention him during a floor speech Monday praising Maguire for his career.

Senate Democrats, meanwhile, have criticized Mr. Trump's selection. Senate Intelligence Committee Vice Chairman Mark Warner, a Democrat from Virginia, said Grenell lacks the intelligence experience needed to lead the nation's intelligence community and accused the president of attempting to "sidestep the Senate's constitutional obligation to advise and consent on such critical national security positions."

The Justice Department has in recent months stepped up enforcement of FARA in the wake of Russia's meddling in the 2016 presidential election.

Paul Manafort, Mr. Trump's former campaign chairman, pleaded guilty to violating FARA after failing to report his work for the Ukrainian government. Federal prosecutors also accused Gregory Craig, former White House counsel for President Obama, of lying to Justice Department officials responsible for overseeing registrations under FARA about his work for the Ukrainian government. He was acquitted in September.

Clare Hymes contributed to this report.