Randi Weingarten declared on Facebook that "Target is OUT this back-to-school season," urging teachers to spend school-supply dollars elsewhere.

In February the American Federation of Teachers said it wanted Minneapolis-based Target to take a public stand against Immigration and Customs Enforcement operations in Minnesota. The union, which Ms. Weingarten heads, adopted a formal resolution directing its 1.8 million members to shop elsewhere and stating its plan to expand the boycott to parents, communities and the AFL-CIO's roughly 15 million members.

One problem with this strategy: Teachers' retirement funds own millions of shares of Target. I spent years managing money. When someone proposes an economic campaign against a publicly traded company, I ask who owns the stock. In this case, the people being asked to boycott are among the shareholders.