But researchers and proponents say that thinking about early childhood programs like public school is a useful frame. As schools close because of the coronavirus outbreak, parents are coming to terms with how much they rely on them as a safe place for children to go. The idea treats preschool as an investment that benefits society at large, as primary and secondary education do. Universal programs tend to have broader political support than those that are means-tested — targeted at people below a certain income. And they are simpler to run because people don’t have to submit paperwork to prove they qualify.

For early learning in particular, researchers say, universal options have educational benefits that means-tested programs like Head Start do not. Achievement gaps between rich and poor children are evident by kindergarten, and classrooms with children from various economic backgrounds have been found to improve children’s learning, particularly for lower-income children.

On his campaign website, Senator Sanders cited this as a reason for his plan: “Our means-tested system has created racially and economically segregated child care and pre-K in this country.”

A recent study of 5,100 4-year-olds from 33 states, using data from the Early Childhood Longitudinal Study, compared those who attended universal pre-K (available to everyone); targeted pre-K (available to families below a certain income); and no pre-K. Universal programs had much larger effects on children’s academic performance and kindergarten readiness than targeted programs, and this was especially true for low-income children. The study ruled out various explanations, like spending per student and children’s alternative care arrangements.

“This leaves open the possibility that the universal nature of the program gets families invested, and holds the programs to a higher degree of accountability,” said the study’s author, Elizabeth Cascio, an economist at Dartmouth.

Europe provides an example: Many countries there provide universal early childhood programs, free or subsidized. In many nations, more than 90 percent of children are in preschool at age 3; in France and Britain, it’s nearly 100 percent, according to the Organization for Economic Cooperation and Development.

In the United States, by comparison, one in three children doesn’t attend preschool. Those who don’t are more likely to be Hispanic, from low-income families or with parents who did not go to college.