U.S. Sen. Kirsten Gillibrand says she will oppose President-elect Donald Trump’s pick for defense secretary, retired Marine Gen. James Mattis, saying she wants to maintain “civilian control of our military.”

Gillibrand (D-N.Y.), the ranking Democrat on the Armed Services Committee, said she will oppose a waiver that Mattis needs to fill the post. A law dating to 1947 requires military members to be retired at least seven years before serving in a civilian position such as defense secretary. The one time a waiver exception was granted was in 1950, when Congress allowed retired Gen. George Marshall to lead the Pentagon under President Harry Truman.

“While I deeply respect General Mattis’ service, I will oppose a waiver,” Gillibrand said in a statement. “Civilian control of our military is a fundamental principle of American democracy, and I will not vote for an exception to this rule.”

A Trump transition spokesman acknowledged Friday that Mattis will need the waiver to serve. Spokesman Jason Miller, in a conference call, didn’t address Gillibrand’s criticism but instead referred to statements from others praising Mattis.

Trump is expected to formally announce Mattis as his nominee on Monday, Miller said.

Sen. John McCain (R-Ariz.), chairman of the Armed Services Committee, has praised Mattis and said he will move to confirm the former Marine “as soon as possible.”

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Meanwhile, the transition team expected to continue a slate of meetings with potential administration appointees and advisers Friday. Among them, former Secretary of Defense Robert Gates, former U.N. Ambassador John Bolton and Sen. Heidi Heitkamp (D-N.D.) will meet with Trump, aides said.