Debt Downpayment Act - Amends Federal law to establish limits, for FY 2000 through 2009, on the face amounts of publicly-held obligations subject to the public debt limit and those whose principal and interest are guaranteed by the U.S. Government, except those issued directly to a Federal fund. Defines a "Federal fund" as a Federal trust fund or Government account to which the Secretary of the Treasury is authorized to issue obligations in respect of public money, money otherwise required to be deposited in the Treasury, or amounts appropriated. Describes the face amount for obligations issued on a discount basis based on whether the obligation is redeemable before or after maturity.

Waives these provisions for: (1) any fiscal year in which a declaration of war is in effect; or (2) for the budget year and the next fiscal year, if real economic growth has been negative for two consecutive calendar quarters.

Excludes the receipts and disbursements totals of the old-age, survivors, and disability insurance program under title II of the Social Security Act (including the Federal Old-Age and Survivors Insurance and Disability Insurance Trust Funds) from: (1) any official statement issued by the Office of Management and Budget (OMB) or the Congressional Budget Office (CBO) of surplus or deficit totals of the Federal or congressional budgets; and (2) OMB or CBO displays of Federal budgetary aggregates.