Mayor Bill de Blasio says he wants 3-year-olds to be included in the city's universal pre-k program. NY1's Lindsey Christ filed the following report.

Universal 3K, a clever new name for a very big promise.

Monday, Mayor Bill de Blasio pledged to expand his signature universal pre-kindergarten program to 3-year-olds.

"We believe across the board, this is one of the best and smartest and more important investments we can make," de Blasio said.

This fall, 3-year-olds in the south Bronx and Brownsville, Brooklyn can start enrolling.

The mayor hopes to expand it citywide by 2021. To do that, he'll have to get Albany and Washington to give him $700 million a year, a very tough sell, especially as the mayor does not have much negotiating power with state lawmakers and President Donald Trump is threatening deep budget cuts. 

"We all know we're facing a complicted fiscal enviroment going foward," he said.

For now, the mayor is pledging $177 million city dollars a year by 2021, enough to fund the program for all 3-year-olds in a quarter of the school districts. 

But financing is not the only hurdle. His pre-k program already has gobbled up many available classrooms, leaving little room for 3-year-olds. 

"We're going to have find space where it isn't obviously availble in some districts, and/or we're going to have to build new space out," de Blasio said.

This is the first major education policy announcement the mayor has made in 18 months. And as he runs for re-election, it allows him to highlight one of his key promises when he campaigned four years ago. Seventy thousand 4-year-olds are now enrolled in pre-k, a build-out that took just two years. Including 3-year-olds will take much longer. 

"It will not be easy. This will unquestionably be harder," de Blasio said.

The mayor cited several studies that he says shows the earlier children begin schooling, the better off their will be academically and socially.

If the program is able to be expanded citywide, officials estimate it would serve 62,000 3-year-olds a year. Classes will be capped at 15 students with two teachers, meaning the city would have to hire 4,500 new educators.