HARRISBURG, Pennsylvania — When word reached the dairy farmers gathered at the Pennsylvania Farm Show that President Donald Trump had signed the Whole Milk for Healthy Kids Act, cheers echoed through the cavernous 24-acre complex packed with farmers, thousands of attendees, and even a few goats, cows, and horses.

No one was more exuberant at the news than Dave Smith, executive director of the Pennsylvania Dairymen’s Association, a trade group founded in the late 19th century that supports and advocates the state’s largest agricultural sector: dairy farming.