Among scientists and educators, there's widespread agreement that early-childhood development is essential for youngsters' long-term well-being. In the U.S., politicians have taken notice. More than half of the candidates vying for the Democratic presidential nomination are advocating universal, federally funded preschool for children under five.

The political appeal of "Pre-K for all" is obvious, but the benefits aren't. A rush to universal prekindergarten risks creating a vast new entitlement that mostly subsidizes upper-income families. The goal should instead be getting more poor kids into high-quality programs.

The U.S. lags far behind other rich countries in preschool enrollment. Only about 66% of 3- to 5-year-olds receive some form of early-childhood schooling, the sixth-lowest figure out of 37 countries surveyed by the Organization for Economic Cooperation and Development.

But previous attempts in the U.S. to offer free universal pre-K have yielded uneven results. Most notably, the beneficiaries of such programs overwhelmingly tend to be poor and minority children; higher-income and white students derive little to no benefits from them.

Instead, a more targeted approach would help. Under the federal Head Start program, the government spends $10 billion each year to provide early-childhood education for poorer families. But that covers just one-third of eligible kids. Congress should work with states to make up the difference, so that free, full-day preschool is available to all children in poverty.

In pursuing this goal, policymakers should be as flexible as possible. Rather than cram 4-year-olds into existing K-12 public schools, for instance, they should allow low-income parents to use vouchers to enroll their kids in privately run day-care centers, as Minnesota's Early Learning Scholarship does, so long as those centers adhere to standards set by local school districts.