The National Education Association (NEA, with 1.8 million members) and the American Federation of Teachers (AFT, with 600,000 members) are politically powerful teacher unions attempting to change America's educational politics. With enormous financial and manpower resources, these unions maintain effective lobbying groups seeking favored legislation and political machines supporting local, state, and national candidates. Teacher unions have become the chief representatives of American education. This paper evaluates teacher unions' political effectiveness. On balance, the unions have been reasonably effective; they have successfully supported mayors, governors, state legislators, members of Congress, and one U.S. President, Jimmy Carter. However, their power has not been absolute. Teacher unions did not become politically active until the mid-1960s, when political action committees supporting local candidates were formed. Teacher unions' power is weakened by their intense organizational rivalry and their differences concerning certain educational, social, and political objectives. The NEA, originally more conservative, has moved to an extremely liberal orientation; the AFT, which long supported a liberal, even radical agenda, has moved slightly to the right of center. Since 1972 both unions have supported only Democratic candidates for President, but by 1980 the NEA and AFT were supporting rival candidates in the Democratic presidential primaries. Generally, both unions have been reactive to contemporary school reform efforts. From the equity wave of the 1960's to the excellence movement of the 1980's, they have been followers, rather than initiators. Although both unions are opposed to competency tests for experienced teachers, vouchers, and tuition tax credits, NEA has become suspicious of the excellence movement, and AFT is more comfortable with it. Various other ideological differences are discussed at length. Included are 46 references. (MLH)