Complex sentence: The Federal Government's power to contract, while originally not set forth expressly and specifically in the U.S. Constitution (or early statutes or regulations), was and is regarded at law as necessarily incidental to the execution of its other powers.
Split into simpler sentences: The Federal Government's power to contract, is not set forth expressly and specifically in the U.S. Constitution. Moreover, the power to contract was and is regarded at law as necessarily incidental to the Federal Government's execution of its other powers.