Bill de Blasio was hounded throughout his mayoral campaign for his signature proposal: universal, full-day prekindergarten, funded by a tax on the rich.

The New York Times called it “an arduous feat.”

Opponent Christine Quinn called it a plan “that every Albany insider has said won’t pass.”

Multiple state legislators called it “unrealistic.”

But early childhood insiders say otherwise.

“There is tremendous momentum around expansion of pre-K in every area of the state, from the governor’s Education Commission, from Obama,” said Betty Holcomb, the policy director for the Center for Children’s Initiatives, a longtime advocacy group. “Increasingly, there has been more of a serious look at how we get it done.”

The skeptics, with hands clamped firmly over their wallets, argue that the benefits of pre-K accrue only to children in very high-quality programs, like the legendary Perry Preschool or the Abecedarian Project, with intensive and expensive resources.

But that’s not the case. “Scientific evidence on the impacts of early childhood education has progressed well beyond exclusive reliance on ... the Perry Preschool and Abecedarian programs,” according to Hirokazu Yoshikawa, a New York University education professor and the lead author of a new study of the evidence base for preschool education.

In big citywide public preschool programs, such as those in Tulsa and Boston, children gained between six months and a full year of additional learning in reading and math, Yoshikawa finds. And in New Jersey, a decade of court-mandated universal pre-K in large, high-poverty districts has also produced strong long-term outcomes, according to “Making Prekindergarten Truly Universal in New York,” a new report from the Center for Children’s Initiatives and the Campaign for Educational Equity.

“Preschool programs implemented at scale can be high-quality, can benefit children from a range of socioeconomic backgrounds, and can reduce disparities among them,” said study coauthor Deborah Phillips.

The argument against expanding pre-K then switches to test scores, with opponents warning that scores between children who attended and didn’t attend pre-K converge in later grades. But tests of 3- and 4-year-olds are unreliable things. Other measures, from hothouse programs like Perry and far less rigorous ones like Head Start, show powerful outcomes for behavior, health, future earnings and educational attainment, and reduced crime and teen pregnancy, the Yoshikawa report shows. Children gain lifetime benefits from pre-K.

The evidence, in other words, is overwhelming. Meanwhile, “there is a real crisis for middle-class families around preschool,” Holcolmb said, as they are increasingly unable to afford private programs.

Overwhelming evidence and critical need make quite a case.

Universal full-day pre-K for 4-year-olds would cost about $340 million a year in New York City, according to the de Blasio campaign. A half-point increase in the city tax rate on those making over $500,000 a year would cover it, with money left over to fund after-school programs for middle school students.

Gov. Andrew Cuomo, an implacable foe of tax increases, surprised Albany insiders in late October by signaling support for de Blasio’s efforts. The mayor-elect was “exactly right” about the importance of pre-K, Cuomo said during a New York City visit, and suggested that funding might be found with a “creative” use of current state resources.

Holcomb’s Center for Children’s Initiatives and Michael Rebell, the architect of the Campaign for Fiscal Equity, which won the state’s disadvantaged school districts some $5 billion in education funding in 2003, just published a “statewide road map” for implementing universal full-day pre-K in New York. It puts the early annual costs for the city at about the same level.

They also show the payoffs. “Research shows you recoup about half the investment very quickly,” Holcomb said, in lower remediation costs. Longer term, she cited well-documented studies that find savings in social costs on the order of $7 for every $1 invested.

In other words, it’s not pie in the sky. Quality universal pre-K is so evidently the foundation for educational achievement for poor and not-so-poor children, its payoffs so indisputable, that the only question worth discussing is how, not if.