The U.S. Constitution, in Article I, Section 6, authorizes compensation for Members of Congress “ascertained by law, and paid out of the Treasury of the United States.” Throughout American history, Congress has relied on three different methods in adjusting salaries for Members. Specific legislation was last used to provide increases in 1990 and 1991. It was the only method used by Congress for many years.Source: Congressional Research Service
The second method, under which annual adjustments took effect automatically unless disapproved by Congress, was established in 1975. …
A third method for adjusting Member pay is congressional action pursuant to recommendations from the President, based on the recommendations of the Citizens’ Commission on Public Service and Compensation established in the 1989 Ethics Reform Act. Although the Citizens’ Commission should have convened in 1993, it did not and has not met since then.
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