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As with the other branches，the U.S. judiciary possesses only those powers the Constitution delegates.The Constitution extended federal jurisdiction only to certain kinds ofdisputes.Article III，Section 2 lists them.Two ofthe most signi？cant are cases involving a question offederal law （all Cases i Law and Equity，arising under this Constitution，the Laws ofthe United States，and Treaties made） and versitycases，or disputes between citizens oftwo different states.Diversity jurisdiction allows each party to avoid litigating his case before the courts of his adversarys state.
A second judicial power emerged in the Republics early years.As explaine in Chapter 2，the U.S.Supreme Court in the case of Marbury v.Madison （1803） interpreted its delegated powers to include the authority to determine whether a statute violated the Constitution and，ifit did，to declare such a law invalid.A law may be unconstitutionalbecause it violates rights guaranteed to the people by the Constitution，or because Article I did not authorize Congress to pass that kind oflegislation.
The power to interpret the constitutional provisions that describe where Congress may legislate is thus very important.Traditionally，Congress has justified many statutes as necessary to regulate commerce among the several States，or interstat commerce.This is an elastic concept，difficult to describe with precision.Indeed，one might for nearly any statute devise a plausible tie between its objectives and the regulation ofinterstate commerce.At times，the judicial branch interpreted the “commerce clause” narrowly.In 1935，for instance the Supreme Court invalidated a federal law regulating the hours and wages ofworkers at a New York slaughterhouse because the chickens processed there all were sold to New York butchers and retailers and hence not part ofinterstate commerce.Soon after this，however，the Supreme Court began to afford President Franklin D.Roosevelts New Deal programs morlatitude，and today the federal courts continue to interpret broadly the commerce power，although not so broadly as to justify any legislation that Congress might pass.