Some people dismiss it as “identity politics.” But the reason leadership diversity matters became crystal clear on the Democratic debate stage Wednesday night. Leaders of color like New Jersey Senator Cory Booker and former San Antonio mayor Julian Castro forced tough questions about criminal justice reform on Vice President Joe Biden (even if the African American Booker also had to answer for police issues on his watch as Newark mayor, as Senator Kamala Harris had to answer for her record as a black prosecutor).

But the high point of a very uneven debate—dramatic, yes, but sometimes ridiculously so—came when Senators Kirsten Gillibrand and Kamala Harris grilled former vice president Joe Biden about his stands on women’s issues over his long career. If Biden is not the Democratic nominee next summer, these will be moments we revisit to explain why.

It began when CNN’s Dana Bash asked tech businessman Andrew Yang about Harris’s proposal to fine companies that pay women less than men. (I’d have preferred a question that got to the bottom of how Harris would figure out companies were doing so, but that’s not what we got.) Yang defaulted to his plan to give every American $1,000 a month—a worthy goal, but one that wouldn’t specifically help women.

Gillibrand dodged the question entirely to go after Biden. She had been foreshadowing her attack on Twitter this week, suggesting that certain Democratic colleagues have been less than supportive of women working outside the home. Some people guessed she was subtweeting Senator Elizabeth Warren, whose groundbreaking book The Two Income Trap did indeed show how American families kept pace with declining wages by sending another parent, usually the mother, into the workforce. But that seemed unlikely; Warren was herself a mother who worked outside the home, for a while as a single parent.

It turned out Gillibrand was referring to Biden, and an op-ed he wrote in 1981 claiming that expanding the federal childcare tax credit would let more women work but subsidize “the deterioration of the family.” He went on: “The day-care centers and nursing homes blossoming across the American landscape are monuments to our growing unwillingness to accept personal responsibility for those to whom we owe the most—our children, our parents and our grandparents.” Since women tend to take care of children and elders, it was clear exactly who Biden believed was “avoiding responsibility,” Gillibrand noted.

So the New York senator asked Biden directly: “I just need to understand as a woman who’s worked my entire career as the primary wage earner.… am I, serving in Congress, resulting in the deterioration of the family, because I had access to quality, affordable day care? I just want to know what he meant when he said that.”

Biden struck back, as he was entitled to, by pointing to his own years as a single father after his wife died in a car accident, which also took the life of his daughter and injured his two young sons. He claimed his real beef was that the tax credit could subsidize well-off families. But Gillibrand didn’t back off. And Biden began with an answer that elicited real sympathy, but deteriorated from there.

“From the very beginning my deceased wife worked when we had children. My present wife has worked all the way through raising our children. The fact of the matter is the situation is one that I don’t know what’s happened.” I think he was asking what happened to his relationship with Gillibrand. Because he went on:

“I wrote the Violence against Women Act. Lilly Ledbetter. I was deeply involved in…the equal pay amendments. I was deeply involved on all these things. I came up with the ‘It’s on us’ proposal to see to it that women were treated more decently on college campuses. You came to Syracuse University with me and said it was wonderful. I’m passionate about the concern making sure women are treated equally. I don’t know what’s happened, except that you’re now running for president.”

This was a bad move, to knock Gillibrand for challenging him even though they’ve been allies. Everyone on that stage has been allied at some point; they’re all Democrats. It summoned up for me the awkward moment at the very beginning of the debate when Biden hugged Harris as they were first introduced on stage and joked, “Go easy on me, kid,” alluding to their clashes at the first debate.

It was a joke; I get that. But given all the criticism Biden has drawn about his old-school treatment of women, it wasn’t a joke he should have made, calling the 54-year-old senator a “kid.” Likewise, he shouldn’t be acting like Gillibrand owes him anything; the two Democrats are running against each other for president. Whether he likes it or not.

At any rate, Bash then asked Harris to respond, and she took an entirely different tack, raising another Biden flip-flop on women’s issues: his very recent decision to support lifting the Hyde Amendment prohibiting federal funding for abortion services, after supporting Hyde since it was passed in the late 1970s.

“I mean, talk about ‘now running for president,’ you changed your position on the Hyde Amendment, Vice President, where you made a decision for years to withhold resources to poor women to have access to reproductive health care, including women who were the victims of rape and incest. Do you now say that you have evolved and you regret that? Because you have only [opposed it] since you’ve been running for president this time.”

Biden sputtered, claiming he favored restrictions on federal funding only “because there was other access for those kinds of services provided privately.” I assume he means charity; yet that would be like opposing the passage of Social Security because charities used to take care of the elderly poor (badly and unevenly, but they were out there). Harris kept hammering him on it until Bash, mercifully, turned to someone else.

The Gillibrand-Harris moment was the deepest any of the debates have gone on women’s issues—and Harris deserves credit for being the first Democrat to bring up the issue of abortion in this round of debates. It was tough to watch Biden squirm. But if he’s going to be the nominee, he’s going to have to answer for his old-school attitudes on women over the years—just as Harris will have to answer questions about her past as a prosecutor, which made her uncomfortable on Wednesday night.

It’s impossible to imagine the debate going into the depth it did—on equal pay, or women’s workplace role, or abortion—if there hadn’t been more than one woman on that debate stage. Over two nights, we saw six. I’m on record hoping that the field narrows before the September debates, but I’m hoping it loses as few women and people of color as possible. There’s a reason that Wednesday night was the superior debate, and it wasn’t just the presence of those particular two front-runners, Biden and Harris (neither of whom had a very good night, for the record). A debate stage with three women and five people of color (out of 10) couldn’t help but produce more passionate genuine debate than the clash of 10 white people we watched Tuesday night. Let’s hope we see much more like this.