As he enters his final weeks in office, Mayor Bill de Blasio is resurrecting an old campaign promise to ban horse-drawn carriages in New York City.When Bill de Blasio first ran for mayor of New York City, he promised to ban horse-drawn carriages “on Day 1.”Eight years later, with just six weeks left in office, Mr. de Blasio is trying one last time to fulfill that pledge.His administration is developing legislation that would phase out the use of the carriages in Central Park and replace them with “show cars,” according to a series of internal City Hall emails marked “confidential” that were sent between late October and last week and reviewed by The New York Times.The promise to ban horse-drawn carriages, along with an ultimately successful plan to implement universal prekindergarten, was among a handful of major proposals that animated Mr. de Blasio’s successful mayoral bid. Mr. de Blasio and some advocates argue that it is inhumane to use horses for transportation in a modern city filled with cars.Now, as the mayor contemplates a run for governor next year, he has returned to his core campaign issues: In an appearance on MSNBC on Thursday morning, he proposed statewide, year-round, all-day school, a vision that he said would “revolutionize education in the State of New York.”Mr. de Blasio has yet to announce his plan to ban horse-drawn carriages, which would require approval by the City Council, but it has been quietly moving forward. In the emails, city officials said they were aiming to have the legislation ready by Dec. 16, when the City Council is expected to hold its last full meeting of the year.Danielle Filson, a spokeswoman for the mayor, said he had always wanted to ban horse-drawn carriages, and that he hoped the City Council would again consider it.The mayor’s office has directed the Economic Development Corporation to contract with a consulting firm, Langan Engineering, to conduct an analysis of the proposal, with a focus on its environmental, transportation, and socioeconomic impacts, according to the emails. The firm’s managing principal did not respond to requests for comment.It remains unclear if there is any appetite in the City Council to ban horse-drawn carriages. “The Council has not received a proposal from the mayor,” Shirley Limongi, a spokeswoman for the Council, said in a statement. “We will review anything we do receive.”The City Hall emails do not define “show cars,” but proponents of banning the carriages have previously pushed to replace them with electric-powered vehicles resembling old-time carriages.In 2018, Appaloosa Management Charitable Foundation, named for a horse breed and run by the billionaire hedge fund manager David Tepper, retained lobbyists to push for such a plan, according to city records and a city official, who was not authorized to speak publicly. Little came of the effort.This April, New Yorkers for Clean, Livable, and Safe Streets, the leading advocates for the ban, retained the lobbying firm Blue Suit Strategies to push Mr. de Blasio to pursue a similar plan, city lobbying records indicate. The organization is paying the firm $7,000 per month.The group, known as NYCLASS, helped fund a campaign to topple the 2013 mayoral candidacy of Christine Quinn, then the City Council speaker and Mr. de Blasio’s rival, in part because she did not support a ban on horse carriages. The campaign was credited with helping to undermine the candidacy of Ms. Quinn, who was considered the early front-runner.In the ensuing years, NYCLASS pushed Mr. de Blasio to fulfill his promise. But efforts to pass legislation went nowhere, including in 2016, when the mayor failed to push through a bill that would have reduced the number of horses on city streets and confined them to Central Park.The group has gotten involved in more recent political efforts. This year, it supported a super PAC that ran ads targeting Andrew Yang’s mayoral campaign after Mr. Yang responded “no” to a questionnaire asking if he supported efforts “to strengthen welfare protections and increase the standards of care for New York City’s carriage horses.”And in October, after a grisly collision between a horse and a car, NYCLASS ran roughly $200,000 worth of TV and digital ads calling for the elimination of the industry.Steve Nislick, the group’s co-founder, said that New York should follow the example of Guadalajara, Mexico, which replaced horse-drawn carriages with electric vehicles.Takeaways From the 2021 ElectionsCard 1 of 5A G.O.P. pathway in Virginia. More