Pool via Getty Images New York City Mayor Bill de Blasio reads to children in a pre-kindergarten class at P.S. 130 on February 25, 2014 in New York City.

When Bill de Blasio ran for New York City mayor in 2013, his plan to enact a universal preschool program for 4-year-olds was treated more like a lofty ideal than a workable promise. American Federation of Teachers president Randi Weingarten suggested the goal was unrealistic, and called the proposal’s early iteration “non-serious.” Experts were skeptical that he could adequately overcome the obstacles to providing pre-K for so many children.

Four years later, de Blasio has successfully implemented a Pre-K for All program that serves about 70,000 kids. But as he gears up to run for his second term, he has ambitious plans to expand the early education services offered to city families. On Monday, de Blasio announced plans for 3-K for All, a program that could roll out free pre-K for all the city’s 3-year-olds by 2021.

It’s a plan that is undoubtedly ambitious for the largest city in the country, and its scope stands out even among other progressive states and municipalities with preschool programs. Yet, unlike when de Blasio first proposed universal pre-K on the campaign trail, this proposal is getting taken seriously, he says.

“When I first introduced pre-K, I can’t tell you how many people told me it was a pipe dream,” de Blasio told The Huffington Post on Tuesday. “The press was openly dismissive,” questioning where the money to pay for the plan would come from, and whether it could serve so many children so quickly, he said.

This time around, neither “political insiders” nor “everyday people” seem flippant about the plan, “even if it’s going to take a lot of work and time.”

.@NYCMayor introduces plan to offer high-quality, full day preschool to all 3 year olds by 2021! #3KforAll https://t.co/eXo3VUGMJ6 — Randi Weingarten (@rweingarten) April 24, 2017

3-K for All will face obvious obstacles ― and it is rolling out at a slow, incremental pace compared to its Pre-K for All predecessor, which took two years. The city already struggled to find space for centers for 4-year-olds, and de Blasio predicts that it will have to “build early childhood centers from scratch” over the next few years. The state budget helped fund the first pre-K program, and the city will need significant funding commitments from the state and federal government to fully enact the new plan as well.

The mayor is optimistic that the city’s efforts on this front will inspire people in both red and blue areas to continue to fight for young children on the local level. There are few indications that Donald Trump’s administration will prioritize early childhood education, and even when Barack Obama was president and pushed the issue, Congress didn’t pass funding for a nationwide pre-K program. Democratic and Republican voters both express support for the issue, though.

“This is something happening to the grassroots and bubbling up all over the country,” said de Blasio. “What we do know is what happens locally, that’s the tangible reality. Local dynamics around the country add up to something.”

Pre-K for all has generally received successful marks, although it has not been without criticism. Preschool classrooms tend to be heavily segregated by race, similar to the city’s K-12 system.

But overall it has succeeded in its goals of giving parents greater early childhood education options. This is a luxury that de Blasio did not have when his kids were growing up, and he worried about whether his kids would get a spot at the competitive preschool center near them. He says the experienced helped spur him to champion this issue.

“I think there’s a real argument here that we got to create a less stressful environment for parents,” said de Blasio. “This is the march of history, and it’s about showing it can happen and work in places all over the country.”