A political party is an organization that seeks to advance the interests of its members by obtaining political power, which is typically achieved by contesting and winning elections. Although a competitive party system is considered a sine qua non of modern democracy, parties operate in many authoritarian countries as well.
Active political parties have emerged in virtually every country that has a democratic constitution. A 1967 study attributed the creation of the existing parties of the western democracies to four "cleavages" that had arisen from national and industrial revolutions; which were identified as centre versus periphery, church versus state, landowners versus industrialists, and capitalists versus workers . Subsequent research covering a range of countries, has indicated that similar party structures had persisted during the rest of the twentieth century . The 1967 study had observed that, with a few exceptions, party structures had become "frozen" in the patterns of the 1920s but later studies indicated that, although most of those patterns had not changed, increasing signs of volatility suggested the prospect of future change .
Although the identities of many of the major parties in the western democracies have remained much as they were in 1920s, their policies have undergone significant change. It has at times been possible to summarise the policy of a European party mainly in terms of left/right positions in a spectrum of alternatives. In one such spectrum, the variable has been the extent of public ownership; in others, the resources to be devoted to public services, or the redistributive effect of taxation. Recently, however, party policies have tended to be more closely definable in terms of issues such as employment protection, immigration, law-and order, and the environment . There has also been a substantial decline in class-related voting in Western Europe  as well as a decline in voter turn-out at elections. In the United States, policy differences between the two major parties have been less clear-cut than in Europe, with a narrower range of left/right positions and a wider range of policy issues , proposals on many of which, however, have had active support from minorities of both parties.
In Germany, the parties are very different from the 1920's. The end of World War II resulted in a very different political scene, abruptly ending the powerful influence of the Nazi Party. Secondly, the Green Party was founded in the late 1970's and the PDS was founded after East and West Germany merged.
Two broad categories of political organization have been identified by Maurice Duverger : elite-based or cadre organizations with limited membership, and strong central control; and mass-based organizations with independent local branches. Cadre-type organization structures are often associated with a "consociational" outlook which attaches importance to reaching policy agreements with competing parties by bargaining and compromise: attitudes that can be observed in the political parties of The Netherlands, Belgium and Switzerland. , and which are conducive to the formation of coalition governments. Party members in mass-based organizations contribute to campaigning and take part to varying extents in candidate selection and policy-making. Party membership has been declining in most European countries and only in Austria and Sweden has their proportion of the population been as high as twenty per cent. In Britain it had fallen as low as three per cent by 1980  , and membership penetration in the newer democracies of Spain, Portugal, Hungary, and Czechoslovakia has always been very low . The political parties of the United States do not fit into Duverger's classification - or any other - because of the virtually unique character of the constitution within which they operate. In no other country except Uruguay are election candidates selected by the votes of party members in primary elections, and the United States constitution has other features, too complex to summarise, that affect party conduct.
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- Almanac of Policy Issues (US)
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