Kirsten Gillibrand has formally entered the crowded competition for the 2020 Democratic presidential nomination with an appeal for Democrats to mount a campaign that answers the question: “Will brave win?”

The senator from New York is a politician in progress. She served in the House as a Blue Dog Democrat who got high marks from the NRA and lots of campaign cash from corporate interests. Now, she’s rejecting corporate-PAC money, championing the Green New Deal and Medicare for All, and highlighting an F-rating from the NRA.

This trajectory has inspired a fair share of skepticism. But Gillibrand is certainly not the only 2020 contender who has recognized that Democrats must evolve to the left, and she has done so by taking some bold stands—such as calling for the elimination of the US Immigration and Customs Enforcement agency. She deserves credit for that. And she deserves credit for being ahead of the curve when it comes to holding the Pentagon to account.

For instance, CBS’s 60 Minutes once described Gillibrand as “the political face of the #MeToo movement.” She earned that title not merely with a controversial call for fellow Senator Al Franken to resign over sexual-harassment allegations, but also through her many years of advocacy on behalf of women who face sexual harassment while serving in the the armed forces.

Gillibrand’s history of working to protect women in the military should certainly be highlighted in her presidential bid. But there is another advocacy with regard to the military that Gillibrand ought to emphasize.

Gillibrand makes a big deal about the importance of maintaining strict civilian control of the military, as it is outlined in a Constitution that sought to chain the dogs of war by requiring that the Congress declare the intent of the United States to engage in military conflicts and by empowering an elected president to serve as commander in chief.

This distinguishes the New Yorker from most Democrats, as became clear during the Trump transition.

In December 2016, when President Trump nominated retired Marine general James Mattis to serve as secretary of defense, Mattis required a waiver from a federal requirement that a military officer must be out of uniform for seven years before taking charge of the Pentagon. Having only retired in 2013, Mattis needed congressional clearance before his nomination could proceed.

Most members of the Senate and House rushed to provide it, as Mattis was generally well-regarded within the military and within the corridors of power in DC. But a few legislators objected. They did not do so out of disdain for Mattis, who many expected to be among the more reasonable members of the new president’s cabinet. They did so out of respect for our history, and concern, as Connecticut Senator Richard Blumenthal noted, about “the precedent that [a recently retired general] assuming this office would set.”

Ultimately, the Senate voted 81-17 in January, 2017, to grant the waiver.

Among the small band of Senate dissenters, the most outspoken was Gillibrand, the ranking member on the Senate Armed Services Committee Subcommittee on Personnel. She had already responded to Trump’s December selection of Mattis with an immediate announcement that “While I deeply respect General Mattis’s service, I will oppose a waiver. Civilian control of our military is a fundamental principle of American democracy, and I will not vote for an exception to this rule.” Gillibrand stuck to that principle. After meeting in early January with Mattis, the senator said: “He has served our country admirably. He is well-regarded as an extraordinary general, and I am very grateful for that service, and I’m very grateful that he’s willing to continue his service for the president-elect. But I still believe that civilian control of our military is fundamental to the American democracy.”

It is easy to rewrite the rules in the moment, to entertain exceptions on the theory that particular requests truly are exceptional. Many Democrats joined Republicans in agreeing to grant the waiver.

The House’s 268-151 vote on the waiver saw a clearer partisan divide, with many Democrats expressing frustration over what was seen as disrespect for the chamber by Trump’s strategists who had canceled a planned appearance by the retired general before the House Armed Services Committee. Their concerns about how Trump and his appointees were relating to Congress were well-founded—as ensuing developments so amply illustrate.

But Gillibrand’s argument for voting against the waiver went much deeper.

Asked to comment on other members of the Senate, many of them Democrats, who refused to join her in opposing the waiver, Gillibrand replied, “You’ll have to ask them. I just think it’s fundamental to the Constitution.”

She stuck to that principled position.

When the Senate Armed Services Committee voted 26-1 to recommend approval of the Mattis nomination, Gillibrand cast the sole “no” vote.

When the full Senate voted 98-1 to confirm Mattis, Gillibrand cast the sole “no” vote.

In a chamber composed of senators who have sworn an oath to “support and defend the Constitution of the United States against all enemies, foreign and domestic” and to that “bear true faith and allegiance to the same,” Gillibrand was alone in keeping faith with the founding premise that the civilian governor of the land should always keep “a watchful and a jealous eye” on military power.