More uncompassionate conservatism: The Bush administration is changing the nation's largest program of housing assistance so that, for the first time, the government no longer is promising to pay the full cost of rent vouchers that help nearly 2 million poor families. The Department of Housing and Urban Development is putting into place the new payment method for the program, a cornerstone of federal housing policy known as Section 8, before Congress decides whether to endorse a broader proposal by the administration that would eliminate many longtime federal rules governing which people get rent assistance and how much they must pay. The payment change, which is infuriating congressional Democrats and advocates for affordable housing, is essentially a different route for the administration to accomplish a central goal of its larger proposal: to constrain rapid growth in the program's spending.