The first Senate Democrat has come out against one of President-elect Donald Trump's more popular cabinet nominees, as Sen. Kirsten Gillibrand of New York says she won't back a special waiver for the general.

'While I deeply respect General Mattis's 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.'

Mattis' wartime experiences as a 'fighting general' and his intellect have won him strong early reviews from Republicans and Democrats, including Rep. Adam Schiff, the top Democratic on the House Intelligence Committee.

He also is seen as a possible check against some of Trump's instincts. Mattis has publicly criticized Russia, and Trump has credited him with raising persuasive arguments against water-boarding, a harsh interrogation technique the president-elect has said should be brought back.

Senator Kirsten Gillibrand of New York said she won't back a waiver for retired General James Mattis to be the Secretary of Defense

Mattis is unlike Trump's other nominees, including some like Health and Services secretary nominee Rep. Tom Price, who have already shown signs of being political lightning rods.

Mattis, however, must contend with a 1947 law that prohibits someone from serving as Secretary of Defense unless they have been out of uniform for at least seven years.

Dealing with the problem would require a congressional waiver, legislation that could be required to meet a 60 vote threshold in the event of a Democratic filibuster.

In a 52-48 Senate, Republicans would be required to pick up eight Democrats to enact the waiver, which would easily pass the House.

However, Gillibrand's statement only said she would 'oppose' a waiver, so it isn't clear how much resistance she might offer. She is the ranking member on the Senate Armed Services Committee Subcommittee on Personnel.

Trump told a crowd of supporters Thursday that he has selected Marine General James Mattis to be his secretary of defense

Senate slowdowns of nominations also can be used as leverage for other matters.

Trump told a crowd of supporters that he has selected Marine General James Mattis to be his secretary of defense, hours after the news broke and his transition team tried to tamp down the story.

'We are going to appoint Mad Dog Mattis as our secretary of defense. But we're not announcing it until Monday so don't tell anybody,' Trump quipped at a rally in Cincinnati as part of a 'Thank You Tour'.