Former Secretary of State Hillary Clinton sounded less like a decades-long supporter of charter schools over the weekend and more like a teachers union president when she argued that most of these schools “don’t take the hardest-to-teach kids, or, if they do, they don’t keep them.”

Her comments in South Carolina came straight from charter school critics’ playbook and distanced her from the legacies of her husband, former President Bill Clinton — credited with creating a federal stream of money to launch charters around the country — and President Barack Obama, whose administration has dangled federal incentives to push states to become more charter friendly.

The change in tone on charter schools mirrors other moves Clinton has made to nail down the support of liberal blocs in the face of the progressive challenge of Bernie Sanders, including her recent decision to oppose the Trans-Pacific Partnership. And like her reservations about free trade, her new rebuke of charter schools suggests she’ll be less willing to challenge core Democratic constituencies than either her husband or Obama.

Teachers unions have been early and enthusiastic supporters of Clinton. American Federation of Teachers President Randi Weingarten, a noted opponent of many education reform efforts, is a longtime friend and informal adviser to her campaign. Unions say they aren’t anti-charter but often attack the schools, a majority of which employ teachers who aren't unionized, accusing them of siphoning off money from traditional public schools.

"Hillary Clinton looks at the evidence. That's what she did here,” Weingarten told POLITICO. “She called out that many charters don’t take the hardest-to-teach kids or don’t keep those with academic or behavioral issues.”

In contrast, the Democratic advocacy group Education Reform Now posted a statement from Director Charles Barone, who wrote that Clinton’s recent comments were “highly disappointing and seemed to reinforce fears about how her endorsements from both major teachers unions would affect her K-12 platform.”

Unions, along with some traditional school administrators and parents, have long charged that charter schools too often reject or push out special needs students or other kids perceived to be troublesome. The issue exploded recently in New York City when high-profile charter operator and former council member Eva Moskowitz conceded that one of her Success Academy campuses was found with a “got-to-go” list naming children considered to be difficult.

Jesse Ferguson, a Clinton campaign spokesman, did not address the evolution in Clinton’s views, but instead said Monday that she had for decades “been a strong supporter of both public charter schools and an unflinching advocate for traditional public schools.”

Clinton won endorsements from the country’s two major teachers unions over the objections of some rank-and-file members who questioned her education priorities, including her long history of support for charter schools. The backing of the AFT and especially the 3-million-strong National Education Association — the nation’s largest union — gave her a welcome boost ahead of the Democrats’ first debate, as Vermont Sen. Bernie Sanders was gaining momentum and Vice President Joe Biden was seriously considering a run.

Clinton has shied away from campaigning on K-12 issues, which tend to divide crucial elements of her party — unions and education reformers. But her newest comments are heartening to Carrie Pugh, the NEA’s political director, who called them “spot on.”

“I do think she is really listening to the practitioners,” Pugh said, adding that, “it’s certainly been an issue that’s come up” during meetings with Clinton and educators in a variety of formats.

Charters use public dollars but are run outside of the confines of many of the rules and regulations that apply to traditional public schools. Numbering more than 6,400 today — or about 7 percent of publicly funded schools — they have become especially popular in urban areas including New Orleans, Detroit and the District of Columbia. Many Republicans embrace charter schools.

When Bill Clinton took office in 1993, there was a lone charter school in Minnesota, according to advocates. He helped create a grant program that provides seed money to open charters, providing hundreds of millions of dollars over the years and paving the way for thousands to open. As first lady, Hillary Clinton publicly applauded the program.

“The president believes, as I do, that charter schools are a way of bringing teachers and parents and communities together — instead of other efforts — like vouchers — which separate people out — siphon off much needed resources; and weakening the school systems that desperately need to be strengthened,” Hillary Clinton said at a 1998 White House meeting.

In 2011, Bill Clinton was honored with a lifetime achievement award from the National Alliance for Public Charter Schools.

Nina Rees, president of the Alliance, noted that Bill Clinton is considered “one of the thought leaders and early adopters of charter schools” and said her organization appreciates past support from the Clintons. But Rees said her organization takes issue with Clinton’s recent statement.

“The data points we have … demonstrate that we are serving the hardest to teach and in fact many of our school leaders are seeking precisely the hardest to teach students and doing a very good job of educating them,” Rees said.

In her 1996 book “It Takes a Village,” Clinton endorsed charters as just the kind of innovation that could overcome stifling bureaucracy and return control to parents and teachers.

“I favor promoting choice among public schools, much as the President’s Charter Schools Initiative encourages,” she wrote. “Federal funding is needed to break through bureaucratic attitudes that block change and frustrate students and parents, driving some to leave public schools.”

Three years later, she encouraged NEA members at a meeting in Orlando to support the charter school movement as a way to improve education broadly, noting the union had helped start some charters.

“And I'm very pleased that you have done this, because I think when we look back on the 1990s, we will see that the charter school movement led by experienced, committed, expert educators will be one of the ways we will have turned around the entire public school system,” she told the crowd. And Clinton noted her support of charters during her prior bid for president, such as during a chat with the Des Moines Register in 2007.

In an answer on an AFT questionnaire this summer, Clinton said charters need to be held to the same standards as traditional public schools. Unions say they want more charters to fulfill their mission to innovate — and they want the schools to be better regulated.

NEA President Lily Eskelsen García said Clinton indicated during meetings that recent research shows which “schools did not take the kids who are in the most challenging situations” and that charter schools were designed to be incubators of innovation — not separate school systems.

“Here’s the thing. She’s not saying anything that she hasn’t said before,” Eskelsen García told POLITICO. “This is very consistent. … By the way, it’s the same thing that if you asked me about charters that I would say, I say show me the charter school.”

Answering a question from commentator Roland Martin on Saturday, Clinton noted her support of charter schools for the last 30 years and said that they still have a place in the American education system.

But Clinton went on to say that “most charter schools” don’t work with the most challenging students.

“And so the public schools are often in a no-win situation, because they do, thankfully, take everybody,” she said, “and then they don’t get the resources or the help and support that they need to be able to take care of every child’s education.”