By Suzanne Gamboa

SAN ANTONIO — Democratic presidential candidate Julián Castro has said the first thing the United States needs to do is to make sure it is the smartest nation on earth.

The way to be the smartest, the former San Antonio mayor and Obama administration housing secretary has been saying in Iowa and other states, is for the nation to start early by investing in pre-K education.

Castro’s signature achievement as mayor or of San Antonio was getting voters to approve a one-eighth cent sales tax increase to pay for a full-day, quality pre-K program. He led the nation’s seventh largest city from 2009 to 2014.

Having made Pre-K 4 San Antonio (known as Pre-K 4 SA) happen, Castro pledged at his Jan. 12 speech announcing his 2020 run that as the nation's chief executive, he would make "pre-K for the USA" happen.

Early childhood education hasn’t generally been a top issue in presidential campaigns. In 2016, college debt relief and a tuition-free college were the education priorities candidates discussed most. Those issues are still around for 2020, along with teacher pay.

But opportunity is a common theme in 2020 candidates’ campaigns: opportunity for economic mobility, for fairer taxation and justice systems, and for access to health care and education.

Students at Pre-K 4 SA's North Education Center learn how to plan their day and carry out their plan.Courtesy of Pre-K 4 SA

While early education is not a gender issue, the presence of more women in the Democratic presidential candidate pool, as well as the progressive shift of the party, have set up a chance for the issue to get an airing as candidates jockey for the party nomination in the primary season.

Last week Sen. Elizabeth Warren, D-Mass., who also is seeking the Democratic nomination for president, unveiled the outline of a $700 billion package focused on child care and early learning to be paid for with a "wealth tax."

Cal Jillson, a Southern Methodist University political science professor and expert on presidential politics, said Castro was on the cutting edge when, as mayor, he pushed for investment in pre-K. As a candidate, he could leverage this to push for early education policies.

“An education package that has pre-K in it is very attractive, particularly to the Democratic voter," Jillson said.

Trying to hit that nerve, Castro told the Democratic crowd at the Story County Soup Supper Fundraiser in Ames, Iowa on Feb. 24, “We’re good in investing in things — in roads, in bridges, in airports, in stadiums, but not oftentimes investing in people.”

'It comes down to political will'

As it stands now, the country is making very slow progress on pre-K education, according to early education expert W. Steven Barnett, senior co-director and founder of the National Institute for Early Education Research at New Jersey's Rutgers University. Some places are moving forward faster than others, such as New York City, as well as Vermont, Wisconsin, Washington, D.C., and Alabama.

There is widely accepted research on the long-term benefits of high quality pre-K, including lower rates of crime and teenage pregnancy, higher earnings and better health, as well as impacts on children’s school readiness and success, according to the “Pre-K in American Cities” report by the NIEER, the de Beaumont Foundation and Kaiser Permanente.

The Centers for Disease Control and Prevention has identified pre-K and other early childhood education programs as impacting and improving child and adult health, such as reducing obesity and fostering motor skill development, the report said.

But less than one quarter of 4-year-olds and a small percentage of 3-year-olds have access to high quality pre-K. High quality is defined by benchmarks set by the NIEER, including teacher education levels, learning goals, class size and health screenings.

The report’s researchers stated that at the nation’s current rate of growth in pre-K education, it would take 150 years to reach 75 percent enrollment and most of that instruction would not qualify as high-quality.

The Learning Policy Institute, an education policy research group, recently released a study on preschool education, including reviews of evaluations on 21 large-scale public preschool programs.

“The main issue is not whether preschool works, but how to design and implement effective preschool programs that deliver on their promise,” the institute’s president, Linda Darling-Hammond, said in a Jan. 31 statement.

For many cities though, the cost to implement high quality pre-K is what makes them hesitate. Some critics question the value of the expense if children go on from high-quality pre-K to subpar schools.

Advocates like to emphasize that there is a return on pre-K investment -- for every $1 invested, there is a $7 return -- and that communities can pay for high quality education now or the costs of an under-educated population later. They also acknowledge that early education is only a part of the education reform needed.

But it's not far-fetched to think universal pre-K could eventually emerge from the community and city systems like PreK 4 SA, Barnett said. It's how the nation got high school, he said.

“It’s not inexpensive, but, say compared to a war in Afghanistan, it’s really cheap,” Barnett said. “I do think it comes down to political will.”

The role the federal government plays in preschool is mostly in programs such as Head Start, which are restricted to low-income families, and in some education funding.

Democrats can point to recent inroads in early childhood education. President Barack Obama made improving early education part of his agenda in his second term and worked with Republican and Democratic local and state officials to foster momentum behind the issue.

Congress failed to provide Obama with the $75 billion from a cigarette tax that he sought, but the administration implemented a competitive grant program to assist communities with early education initiatives, said Eric Waldo, executive director of Michelle Obama’s Reach Higher program and former deputy chief of staff in the U.S. Education Department.

Cecilia Muñoz, Obama's former director of the White House Domestic Policy Council, said that despite the lack of funding, his administration built a coalition of officials around the country and managed to expand access to pre-K in 30 states.

“I think it’s critical for Democrats to make sure education in general has a larger place on the national stage,” Waldo said. “It’s a great opportunity for candidates to be forward looking in cradle-to-career proposals. If you have a leader who does that, well, that can give them an advantage.”

On pre-K, 'quality matters'

Seconds after Enrique Martinez, 41, of San Antonio stepped into a Pre-K 4 SA classroom, his son, Enrique, 4, jumped into his arms. A father of three who served in the military, Martinez became emotional as he discussed how preschool has benefited his son.

In his previous child care, said Martinez, his son was given small materials to read but there was "a lot of playing, a lot of distraction and stuff and I don’t like that.” Now, he asks his son, “what’s 2 + 2 and he bursts out with 4.”

Enrique Martinez chats with his son Enrique, 4, as he picks him up at the Pre-K for SA South Education Center in San Antonio.Suzanne Gamboa / NBC News

“I’m setting him up for success,” Martinez, who is now a stay-at-home father, said. “I want him to have the best education. I want him to be focused on education.”

It wasn’t easy to get the tax increase that supports Pre-K 4 SA passed in conservative Texas, a point Castro is emphasizing on the campaign trail.

“What I had seen was that San Antonio ranked a lot lower than most places, most communities of its size when it came to individual achievement, and that we needed to turn that around,” Castro said.

The tax generates $35 million a year that is distributed among the city’s 15 school districts for pre-K. Eight districts partner with Pre-K 4 SA so that some of their share of the money helps pay for full-day enrollment.

"A lot of programs start with access." said Sarah Baray, Pre-K 4 SA's CEO.Suzanne Gamboa / NBC News

The program is not universal pre-K, but enrollment is open to low-income students, English Language Learners, children in military families, those who are in foster care or who are homeless. The state provides funding to school districts for half-day pre-K that sets similar eligibility standards.

The program's objective has been to create a high quality preschool with well-trained teachers and a curriculum that helps get students ready for school. Its centers are in a sense incubators, where they are trying to create a model for how to build high quality pre-K.

“A lot of programs start with access. They just want to give 4-year-olds or 3-year-olds access to pre-K, and as we say, mediocre pre-K doesn’t change anything,” Sarah Baray, Pre-K 4 SA’s CEO, said.

Though it still has its critics, the program has received good marks in independent reviews.

The first alumni of Pre-K 4 SA reached third grade last school year — the grade level when Texas students take their first state standardized tests in reading and math.

PreK-4 SA students achieved the state average on reading scores, though students of similar demographics usually have below average scores. On math, the PreK 4 SA students exceeded the state average scores.

PreK-4 SA "created a surge of awareness about the value of early childhood education, expanding the number of students enrolled in prekindergarten across San Antonio," the reviewers with the Urban Education Institution at University of Texas at San Antonio said.

Making it a national issue

According to the latest data analysis from NIEER, more states are expanding access to publicly funded pre-K programs, but they aren’t investing enough money in them.

In the 2016-17 school year, 1.5 million children nationally were in state-funded pre-K programs, about a third of the country’s 4-year-olds and 5 percent of 3-year-olds. Funding was about $5,008 per student in the last school year, slightly down from 2015-16.

Even as states and communities try to get more children into pre-K, income disparities are widening. High-income families are investing more in their children, so disparities between what is being accomplished in public pre-K versus privately funded pre-K is growing, Barnett said.

Barnett said there is room for early education to be a forefront issue in the presidential election as an economic competitiveness issue, in addition to being part of the education discussion.

“It’s part of the story about the rest of the world leaving us behind,” Barnett said. “We were once an international leader in education and now we’ve got Korea with double the percentage of our kids going to college. You have Canada as the best educated country in the world, and part of the reason we’ve fallen behind is lack of investment at both ends around our K-12 system.”

Even with the restricted role the federal government plays, early education is an issue that Democrats, especially candidates of color, should talk about, Jillson said. Presidential hopefuls who talk about education and health care are talking to voters “where they live.”

FOLLOW NBC LATINO ON FACEBOOK, TWITTER AND INSTAGRAM.