WASHINGTON — The ranking Democrats of six House congressional committees asked the Pentagon on Wednesday for information about President Trump’s national security adviser, suggesting that he may have violated the Constitution’s emoluments clause by accepting a fee for speaking at a 2015 Moscow dinner.

Michael T. Flynn, a retired Army lieutenant general, sat with Russian President Vladimir Putin during the event, a celebration for Russian Television.

The lawmakers suggest that the fee he received may have violated the Constitution’s emoluments clause, which prohibits top officials from receiving payments from foreign governments.

“It is extremely concerning that General Flynn chose to accept payment for appearing at a gala hosted by the propaganda arm of the Russian government, which attacked the United States in an effort to undermine our election,” the members wrote in the letter.

The letter, signed by the ranking Democrats on the House Intelligence, Oversight, Armed Services, Judiciary, Foreign Affairs and Homeland Security committees, requested any information the Pentagon had about “how much General Flynn was paid for his dinner with Vladimir Putin, whether he received additional payment from Russian or other foreign sources, and whether he sought the approval of the Department of Defense or Congress to accept these payments.” The speech was made during a celebration dinner for Russian Television.

The White House and the Defense Department did not respond to requests for comment on the congressional letter. But a former White House ethics counsel, Norman Eisen, said the questions are “troubling and important.”

“Concern about the alleged ties between the Trump administration and Russia, and its effect on our national security, is precisely the kind of worry that led the Founders to include this prohibition in the Constitution,” said Eisen, who served as ethics adviser to President Barack Obama and who has emerged as a sharp critic of Trump administration ethics policy.

In the past, the Pentagon has advised retiring officers that, because they are subject to recall to military service, they may be subject to the Constitution’s emoluments clause.

“If Lt. Gen. Flynn is subject to recall [by the Defense Department], and accepted foreign government funds without congressional consent then that is a foreign emolument and so a violation of the Constitution,” Eisen said, adding: “Nor would it be a merely technical one.” The Department of Defense has warned retired officers against accepting “indirect payments, including from pass-through companies in the U.S.” in retirement, the congressional letter said.

Flynn has been among the more controversial of Trump’s appointees not only for his trip to Russia but because he has made inflammatory statements in previously published tweets. In November, for example, he tweeted that fear of Muslims is “RATIONAL: please forward this to others: the truth fears no questions . . .”

He was removed from his command of the Defense Intelligence Agency in 2014 because of concerns over his leadership style. While working for the campaign, his firm was also paid as a lobbyist for a consultancy founded by a Turkish businessman.

The permission may be important, the members wrote, because the Constitution prohibits any person “holding any Office of Profit or Trust” from accepting gifts or payments from any foreign country. The Department of Defense has made clear this restriction applies to retired military officers, who continue to hold offices of trust.

The letter, signed by Reps. Adam B. Schiff, D-Calif., Elijah E. Cummings, D-Md., and others, quotes Defense Department instructions to retired officers that says that permission is required. “Significantly, retired regular military officers are also subject to the Emoluments Clause because they are subject to recall, and, therefore, hold an ‘Office of Profit or Trust’ under the Emoluments Clause,” the letter notes.