U.S. Sen. Richard Blumenthal, D-Conn., a member of the Armed Services Committee, said Monday that he had doubts about waiving a rule barring President-elect Trump from naming former General James Mattis, or any other military officer, as defense secretary so soon after retirement.

To reinforce the tradition of civilian control over the U.S. military, former officers are barred from serving as defense secretary within seven years of ending active duty – a rule last waived after World War II to allow George C. Marshall to serve as secretary of state and defense secretary.

“Civilian control over the Department of Defense is a bedrock principle. The standard is a high one,” Blumenthal said. “Gen. Mattis has the burden of meeting it, which he has not yet done. I would vote to waive it only under the most unique and exigent circumstances.”

Mattis retired from the Marines in 2013. U.S. Sen. Kirsten Gillibrand, D-N.Y., already has said she is unwilling to waive the seven-year rule. Blumenthal said he is waiting to hear Mattis talk about his views on civilian control of the military.

Blumenthal commented on Mattis at a press conference in Hartford to amplify the call made by other U.S. Senate Democrats to demand that Trump’s cabinet nominees submit their tax returns, something Trump refused to do during the campaign for president.

“These nominees are highly successful. There are at least three billionaires among them. Almost all are millionaires. They have ties to special interests and corporations, some of them foreign governments and special interests,” Blumenthal said. “The president-elect said he would drain the sawmp, that he woud increase the credibility and trust of the United States government. The best way to ensure greater crediblity and trust for his cabinet is to require full disclosure of their tax returns.”

In a memo for campaign supporters obtained by The Associated Press, Trump’s transition team called the Democratic calls “a PR stunt with zero precedent designed to arm opposition researchers.”

Democrats weakened their own ability to block cabinet nominees in 2013 by eliminating the 60-vote requirement to end a filibuster over the confirmation of executive branch and judicial nominees. A simple majority is now required, and the GOP holds a 52-48 advantage. The 60-vote rule still applies to confirmation votes for the U.S. Supreme Court.