Workers at an animal protection group want a union. The CEO, not so much.

The dispute is not about money, but about social justice and workplace culture at the Animal Legal Defense Fund.

The Animal Legal Defense Fund can say, with little fear of contradiction, that all its clients are innocent. It has sued a California dairy farm, alleging that Dick Van Dam Dairy treated cows and calves cruelly. It has sued the owner of an eight-year-old horse named Justice, accusing her of neglecting the animal. It has served notice that it intends to sue a Pennsylvania roadside zoo that is confining wild animals, including a ring-tailed lemur, black leopard and gray wolves,. For four decades the nonprofit ALDF has pioneered the field of animal law, using the courts to go after people who abuse animals.

Now the tables have turned. A majority of the 70 or so staff members at the ALDF have signed up to form a union, putting the organization’s leaders on the defensive. The union, called ALDF United, has affiliated with a small but fast-growing union called the Nonprofit Professional Employees Union (NPEU), which, as its name suggests, represents professionals at nonprofits. In December, ALDF United filed for an election with the National Labor Relations Board after Stephen Wells, the nonprofit’s executive…