According to the Wagner Act (NLRA), an employer must recognize and treat a union as the representative of all employees (even those that did not vote for the union) if the union has majority support of the employees.

Wagner Act

Also known as the National Labor Relations Act, it's a foundational statute of US labor law which guarantees basic rights of private sector employees to organize into trade unions, engage in collective bargaining, and take collective action such as strikes.

Employer

An individual or organization that hires and pays people to perform work or provide services.

Union Representative

An individual elected or appointed to represent union members in discussions, negotiations, or disputes with the employer.