Kevin Huffman is a fellow with New America and served as commissioner of education in Tennessee from 2011 to 2015.

A Tennessee study found that students who attended the state’s pre-k program did worse by third grade than students who had been lotteried out. (iStock)

As campaign issues go, promoting preschool for poor kids is about as close to a no-brainer as it gets among progressives.

Indeed, when Hillary Clinton officially launched her campaign last summer with a call for expanded access to prekindergarten, the New York Times reported, “Of all the issues Mrs. Clinton could have delved into, early childhood education is perhaps the most obvious and among the safest.”

Both Clinton and Bernie Sanders have made universal, school-based pre-K a centerpiece of their platforms. Meanwhile, they’ve demonized any opposition. “They aren’t just missing the boat on early childhood education,” Clinton said, “they’re trying to sink it.” Sanders, not to be rhetorically outdone, claimed that “to turn our back on children at that period is disgraceful.”

And why shouldn’t we all fall in line on this issue? We know that children from low-income homes enter kindergarten already significantly behind their wealthier peers. Research shows that they hear about 30 million fewer words, they have significantly lower exposure to books, and their impulse control and self-regulation — often called executive function — tend to be less developed than in higher income children. So it makes absolute sense to look for meaningful interventions between birth and age 5.

Unfortunately, the predominant remedy advocated by those on the left is neither as effective, nor cost-effective, as people tend to think.

Researchers at Vanderbilt University have spent the past six years comparing cohorts of Tennessee pre-K students with their peers who applied to the statewide pre-K program but were lotteried out. The results are not stellar. The pre-K students entered kindergarten with a decided advantage over the comparison group, but that advantage diminished over time. By the time the children reached third grade, the pre-K attendees actually underperformed the comparison group.

Welp.

As the state’s education commissioner from 2011 to January 2015, I can’t help being disappointed. Low-income children in Tennessee struggle in school, and like many, I have been hopeful that the school-based pre-K program would boost achievement and, eventually, be expanded.

Still, I’ve been surprised with how pre-K advocates rushed to defend their sacred cow.

Some, including Nobel Prize-winning economist James Heckman, have dismissed the unwanted results as a product of flawed methodology. This is odd given the impeccable credentials and the extraordinary care of the researchers. In fact, based on my multiple conversations with the Vanderbilt team, I would posit they were genuinely hoping for good results and were surprised and discouraged by what they found, but nonetheless committed to honesty in their analysis.

In another defensive maneuver, some pre-K advocates have suggested the reason Tennessee’s pre-K isn’t working is because Tennessee isn’t doing it the right way. “If your program isn’t very good, you can’t expect it to have long-term benefits,” sniped the director of the National Institute for Early Education Research.

That’s funny, because pre-K advocates for years told me how great the Tennessee pre-K program was based on their own metrics. We were in the upper tier of states, meeting nine out of 10 quality standard benchmarks on a well-regarded rubric from — guess who? — the National Institute for Early Education Research.

I understood, though, that Tennessee’s pre-K was roughly analogous to all of its schooling. Like most states, we have some good programs, some bad and a large smattering of average. (We score slightly below average on national tests, though scores are climbing faster than in most other states.)

The studies showing that pre-K “works” are based on small, high-quality pre-K programs. Indeed, if you parse the language of pre-K advocates, you will often find the words “high-quality” when they describe what we need. See, for example, Clinton’s campaign promise that “every 4-year-old in America [will have] access to high-quality preschool in the next 10 years.”

That’s great, but what leads us to believe that we can take small, high-quality pre-K programs and blow them out into a statewide or nationwide intervention? Why would we think we can build a “high-quality” program for all the nation’s 4-year-olds when decades of effort have failed to produce universal high-quality in any other grades?

This matters because, as the Vanderbilt study shows, an average pre-K program doesn’t seem to have academic impacts.

It’s important to note a couple of caveats here. The study is ongoing, and we don’t know how results will ultimately wind up. Also, there is good evidence from other studies that pre-K programming has a positive impact on non-academic results. Pre-K can improve executive function, eventually resulting in better behavior and even higher graduation rates. The Tennessee study is too preliminary to measure these effects.

But while we wait for a full accounting, given the doubts the study has raised so far, and given the urgency of improving early childhood interventions, let’s park the highly politicized groupthink on pre-K and have a real conversation about solutions.

We should consider a wider range of interventions. I would love to see apolitical research comparing school-based pre-K, private day care, church preschool and Head Start. In the same way that public charter schools have become an important addition to traditional public schools, are there ways to let dollars flow to early childhood programs based on effectiveness?

Instead of advocating for universal school-based pre-K, let’s advocate for 3- and 4-year-old children and be open to different possibilities. If our goal is to help them rather than simply to add another grade level onto our public schools, then let’s stop demonizing opposition to pre-K, attacking the bearers of bad news and making pre-K just another tool of partisan orthodoxy. We owe it to our low-income families and the schools that ultimately serve them.