For politicians, improvements to parks, libraries, and recreation centers are as wonderful as happy babies, puppies, and kittens.

They get to pose with children on slides and swings as well as job seekers cruising the web for work in remodeled libraries. And, they cut the ribbons on projects that make residents shower them with gratitude.

But in Philadelphia, the biggest public-works project in years has hit a snag.

City Council is holding up approval of Rebuild Philadelphia because, it says, the $500 million program to refurbish libraries, recreation centers, and playgrounds doesn't diversify the Building Trades unions enough. Rebuild has been touted as a way to alter the unions' pathetic record of inclusion. The Kenney administration says it worked out an unprecedented diversity deal with the Building Trades unions  —  including a target to have 45 percent of the workforce be minorities with a path to union membership — but Council critics say it's too vague to enforce.

Clearly, the Building Trades have historically failed to enlist minority members. The last time they opened their books was 2008 during the Convention Center expansion project. Back then, blacks accounted for less than 10 percent of the memberships of most unions. The current racial breakdown is a closely guarded secret. A spokesman for the Building Trades did not respond to a request for information.

Council, however, can force the Building Trades to keep their diversity promise. Every year of the Rebuild program Council will get to approve spending, and it should use that approval power to make sure the unions are sticking to the deal.

But Council should act to get this program moving because long-neglected neighborhood facilities are only getting worse. Already, the funding, based on the soda tax, is at risk if the courts declare the levy unconstitutional.

A few days ago, Mayor Kenney got tired of waiting for Council. In a letter to Council President Darrell L. Clarke, he said he'd fund initial work by using the city's capital budget. Kenney argues that if this work doesn't get started now, residents will suffer. "We are at risk of losing a summer of construction for facilities that are in dire need of improvements and may not be able to operate next year without them," according to the mayor's letter.

For now, about $18 million in capital budget funds will cover design work and small repairs like fixing leaky roofs and windows while everyone waits for a state Supreme Court decision on whether the city can use the 1.5 percent sweetened-beverage tax to cover more expensive work. The court heard arguments May 15.

Eventually,  the mayor will need Council to fully fund the whole program.

Meanwhile, he seems determined to keep Rebuild on track. On Wednesday, he took key administration figures to the Vare Recreation Center at 26th and Morris Streets to talk about work to be done at the popular site along with the district councilman, Kenyatta Johnson.

More such announcements are being scheduled,  Council should get on board. Otherwise, members are going to have to explain to angry residents why they didn't deliver the refurbished parks, libraries, recreation centers, and playgrounds they said they would.