As the chart shows, congressional Democrats as a group are more liberal than Republicans although well shy of a strong liberal rating of 100. Over time the ideological leanings of both partisan groups fluctuate. The civil rights agenda that dominated the congresses of the 1950s and early 1960s coincided with a spike in liberalism among Republicans in the late 1950s and increasing conservatism among Democrats from about 1960. As of 2005 Democrats and Republicans – with Republicans in the majority as the background indicates – were at their most ideologically polarized since World War II, a trend that began in the early 1970s.
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Many groups rate legislators' agreement with their agenda based on legislators' votes on selected pieces of legislation. The liberal group Americans for Democratic Action (ADA) has rated members of Congress annually since 1947. These ratings serve as a widely used measure of the liberalism or conservatism of individual members, state and regional delegations, and Congress as a whole. A rating of 50 means that a legislator or group of legislators agrees with the ADA agenda about half the time. To the extent members of Congress effectively represent their constituents these ratings also reflect citizens' ideological leanings.