Article 1, Section 8, Clause 2
William Rawle, A View of the Constitution of the United States 81 1829 (2d ed.)
When this Constitution was formed, the United States were considerably indebted to foreign nations, for the expenses of the war, and its own citizens had heavy claims, as well on the Union as on individual states, for services and supplies during the same eventful period. To combine and consolidate these debts, to discharge some and secure the rest, was necessary for the public faith and interest both abroad and at home. But to avail itself of the power of taxation, in order to accomplish such extensive objects at once, would have been injurious to the community. It was foreseen that many public creditors, whether distant or domestic, would be satisfied with the assumption or recognition of the principal and the payment of the interest. By the terms thus introduced, congress received power to make the necessary provisions for such objects. In case of future exigencies, the expenses of war or the failure of part of the usual revenue; a similar mean of continuing the operations and the character of government is also thus provided.
The Founders' Constitution
Volume 2, Article 1, Section 8, Clause 2, Document 9
The University of Chicago Press
Rawle, William. A View of the Constitution of the United States of America. 2d ed. Philadelphia, 1829. Reprint. New York: Da Capo Press, 1970.