Final answer:
The power of Congress most frequently contested in the courts is its ability to regulate interstate commerce, as outlined in the commerce clause of the U.S. Constitution. This has led to varying interpretations by the Supreme Court over time, affecting the scope of Congress's regulatory powers.
Step-by-step explanation:
The power of Congress that is most frequently contested in the courts relates to its ability to regulate commerce among the various states. This power, derived from the so-called commerce clause found in Article I, Section 8 of the U.S. Constitution, has been a point of judicial interpretation and contention.
Historically, the Supreme Court has swung between broad and narrow interpretations of this clause, which has affected Congress's scope of regulatory power. For instance, the landmark case Marbury v. Madison established judicial review, which empowers the courts to declare laws unconstitutional. This review can lead to the Congress's commerce regulation powers being contested in cases like United States v. Lopez and decisions regarding the Affordable Care Act.
The understanding of the limits of Congress's power to regulate commerce has evolved over time and still prompts debate as to whether Congress should maintain broad regulatory power or if the Supreme Court should impose tighter restrictions regarding the commerce clause to diminish potential overreach.