The purpose of this article is to make an in-depth examination of the relationship between labor unions, especially the AFL-CIO, and the Democratic Party in the past one hundred years. Aside from the introduction (Section One), the main body is divided into three sections. Section Two is about the historical background of labor unions' engagement in political activities. It describes briefly, in chronological order, the six stages in the development of relationship between labor unions and the Democratic Party. Section Three analyzes how labor organizations, as special interest groups and pressure groups, use electioneering and lobbying to actually take part in political activities. It also tries to make a preliminary evaluation of their performances and assesses the causes of their successes and failures. Section Four discusses legal problems concerning labor unions' participation in political activities. It details numerous limitations imposed by federal election statutes on political contributions and expenditures by these organizations and the ensuing constitutional implications, especially the First Amendment issue of the protection of freedom of speech. It also examines whether labor unions can utilize the so-called union security clause, contained in the collective bargaining agreements, to use collected membership dues or agent's fees and engage in political activities which are opposed by their dissident members (or non members). Finally, this article concludes that due to various adverse elements, political influences of American labor unions have been declining considerably over the past twenty years. In order to safeguard their existing interests, they have not only to strengthen their cooperations with the Democratic Parry, but also to rally the support of other special interest groups. Under such circumstances, their future participation in political activities will inevitably be much more aggressive and intensive, although they are currently almost totally on defense on all fronts.