WASHINGTON (AP) — Treasury Secretary Timothy Geithner is strongly criticizing a Republican proposal that would prioritize interest payments on the nation's debt and cut spending rather than raise the borrowing limit.

Geithner says in a letter addressed to Sen. Jim DeMint, a South Carolina Republican, that the idea is "a radical and deeply irresponsible departure" from previous practices by presidents of both parties. The letter was copied to 16 other Senate Republicans, including Minority Leader Mitch McConnell.

The debt limit is the amount the government can borrow to finance operations. The United States reached its $14.3 trillion borrowing limit in May. It could default on its debt if it doesn't raise that limit by Aug. 2.

Republicans say they won't support an increase without a plan for cutting spending by an equal amount.