Michelle Rhee was chancellor of the D.C. public schools from 2007 to 2010. She is founder and chief executive of StudentsFirst, a national nonprofit education advocacy group based in Sacramento.

I’m a Democrat because I believe in the party’s basic principles, particularly the idea that we have to look after one another and stand up for those who need help. I believe in fighting for the civil rights of all Americans, especially children and those facing injustices. That’s why I was heartened to see Chicago Mayor Rahm Emanuel advocating for the rights of kids in his standoff with the Chicago Teachers Union. Although his stance made perfect sense to me, it surprised many political observers. After all, Emanuel is a favorite within the Democratic Party, and teachers unions have long been allied with the party.

Emanuel went head to head with the union to get a better contract for the city’s schoolchildren. In the process, he underscored a transformation in the Democratic Party. Increasingly, those who staunchly side with unions at any cost appear to be in the minority, while more Democrats are saying we have to look at education differently.

In Connecticut, Gov. Dannel Malloy (D) pushed through a law bringing more accountability into schools over early and strong union objections. In Los Angeles and Cleveland, Democratic mayors have implemented strong education reforms. As a group, the U.S. Conference of Mayors passed a resolution (led by a Democrat — my husband, Sacramento Mayor Kevin Johnson) supporting parent “trigger laws” and rigorous teacher evaluations based largely on student achievement growth.

Behind the shift is a desire by average Democrats to support common-sense reforms in children’s best interest. But like any shift in alliance, the changing political landscape isn’t without controversy.

Teachers unions have been helpful to many Democratic candidates who would otherwise be outspent by better-funded Republicans. What’s more, those who identify as liberals or progressives — myself among them — tend to be passionate about the rights of workers to stay safe on the job and earn a decent living. But we also are passionate about the rights of all young people, regardless of the circumstances of their birth, to get a great education.

It’s no longer acceptable to ignore the inequities and overall shortcomings of our public education system. Consider that only about half of black and Hispanic students earn high school diplomas with their peers, compared with three-fourths of whites, and that the academic achievement gap between poor students and their wealthier peers is widening. When kids do make it to college, roughly a third need remedial work because they weren’t adequately prepared by the K-12 system. The United States is also falling far behind our global competitors in math and science.

In Chicago, where 80 percent of children aren’t performing at grade-level proficiency standards, according to the Nation’s Report Card, Emanuel simply said enough is enough and stood up for sensible changes aimed at helping kids make much more academic progress.

At the heart of the debate was a focus on teacher quality. We know that great teachers can have a tremendous impact. Yet we don’t have policies in place to ensure that all children are taught by great teachers. That’s why the mayor pushed for an evaluation system that would help determine who is excelling, who needs help and who may be better off in another profession. I’m so glad that, for the first time in Chicago, teacher evaluations will at least consider whether students are learning over the course of the year. But I do wish the union hadn’t won some key concessions.

Despite the mayor’s efforts, the contract wasn’t a home run for kids. It still allows teacher seniority to trump teacher effectiveness in some cases when layoffs unfortunately arise. And under the new evaluation system, less than a third of a teacher’s evaluation will be based on an objective look at how much academic progress his or her students are making. That’s not enough. Educator evaluations ought to consider several factors, but the degree to which our children succeed academically must be what we emphasize.

I also think it’s a mistake to include a third-party appeals process for poor evaluations. That’s not necessary if you have a robust and fair evaluation system, and it will continue to make it overly burdensome and bureaucratic for principals to replace staff failing to meet students’ needs.

One of the mayor’s proposals that was ultimately rejected was a performance-pay system for teachers like the one in Washington, which offers huge pay increases to effective teachers, making them — deservedly — among the best paid in the country.

The strike advanced the national conversation about education reform but did so at a high price: Chicago’s children lost roughly  18 million collective hours of learning time; moms and dads across the city lost wages, and possibly risked jobs, so they could care for their kids; and some children went without the hot meals they reliably get at school.

It was frustrating to hear Chicago Teachers Union President Karen Lewis say toward the end of the dispute that the strike would continue to see whether there is “anything else they can get.” But at least that was clear evidence that, for union leaders, this strike was never about what was best for kids.

Going forward, I suspect more Democrats will say, as Emanuel and President Obama have, that it makes sense to look at how much children are learning when assessing a teacher’s work and to empower parents to help turn around schools that are failing their kids, and that it is right to pay teachers more but to also hold them accountable for results.

I know that opposing unions on some of these policies isn’t easy. But the more that unions continue to attack fellow Democrats, casting everyone who challenges their policies as “anti-teacher” or “anti-union,” the more they isolate themselves from the broader Democratic Party.

Ours is the party that understands that the greatest equalizing force we have in this country is a high-quality public education system through which people can escape poverty.