SAN FRANCISCO — Earlier this month, a federal judge in Washington, D.C., said most American employers must inform employees of their right to join a union and bargain collectively. That information comes by way of a poster, to be posted in lunch and break rooms, next to notices of maternity leave and minimum wage requirements.

The rule was passed by the National Labor Relations Board, a five-member agency charged with enforcing the 76-year-old National Labor Relations Act, which has taken on new urgency under President Obama. Labor and employment lawyers see the rule — and its ratification — as another example of an aggressive effort to shore up labor.