Labor unions in the United States have four functions: recruit new members, negotiate with employers, occasionally conduct strikes to achieve their purposes, and engage in politics by supporting political candidates who are favorable to them and by working to influence legislation. A labor union is an organization of employees whose purpose is to bargain with an employer or a group of employees over pay and working conditions. Union membership is higher in Europe than in the United States. Total union membership generally has been growing in the United States since the 1960s, principally through the growth of public employee unions. However, the percentage of all U.S. workers enrolled in unions has been declining.