AT ONE point during Tuesday’s debate over paid family leave legislation, D.C. Council member David Grosso (I-At Large) said he had some questions about a substitute proposal. Council member Jack Evans (D-Ward 2) gamely tried to answer but was cut off and talked over by an increasingly agitated Mr. Grosso. It was clear the council had already decided, and so never mind the facts showing there might be a better, faster and more responsible way to provide benefits.

Given the council’s 9-to-4 vote to approve the Universal Paid Leave Amendment Act of 2016, it may be difficult for Mayor Muriel E. Bowser (D) to find a fifth vote to sustain a veto, but she should try. The legislation passed by council members — imposing a new tax on employers to fund an expensive paid-leave program while creating a sprawling new government bureaucracy — is a bad bill that, as was elucidated by the city’s independent chief financial officer, carries enormous risks for the city. What makes the move even more mind-boggling is that most of the benefits of the program will go not to D.C. taxpayers, but to Maryland and Virginia residents who work but pay no taxes in the District.

It is telling that even some council members voting for the bill expressed misgivings. “Ill considered” was the judgment of Mary M. Cheh (D-Ward 3), who meekly went along with the majority after a measure she co-sponsored with Mr. Evans failed. “A serious bill with serious consequences to our business community,” said Kenyan R. McDuffie (D-Ward 5), noting there are a lot of issues that still need to be worked out. No kidding — such as, for starters, finding the money to design and build a system for which no model exists. Or figuring out how a city government that sometimes struggles to provide basic services is going to administer the complexities of this program. The D.C. small-business owner who said he gets “chills and nightmares” just thinking of the city administering his employees’ benefits had it exactly right.

The alternate proposal fashioned by Mr. Evans would have required businesses to provide the same amount of leave, with smaller businesses getting tax credits and hardship exemptions. It would present fewer risks to the city and could be implemented quickly. That, though, did not serve the interests of union advocates who wanted a bill they hoped would serve as a national model, and so the more sensible alternative was given short shrift.

Ms. Bowser has been too timid in her approach to this issue. While she made no secret of her reservations about the council’s approach, she failed to mount an effective defense against it. It is not enough, as she has announced, that she won’t attach her name to the bill. She must let the city know through her veto just how unacceptable it is and challenge the council to act in defense of the interests of D.C. taxpayers.