If budgets have to be approved by Congress, why does the Congress again have to approve an increase in borrowing limits?

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Question: 

If budgets have to be approved by Congress before they get signed by the President, why does the Congress again have to approve an increase in the borrowing limits?

If govt debt is at $10 trillion, and Congress approves a budget with a budget deficit of $1 trillion, isn't it implied that the administration can implement the budget only by borrowing an extra trillion?

Answer: 

Just to be clear, “the debt limit is the total amount of money that the United States government is authorized to borrow to meet its existing legal obligations, including Social Security and Medicare benefits, military salaries, interest on the national debt, tax refunds, and other payments.” Debt limit and borrowing limit mean the same thing. And yes, the “debt limit does not authorize new spending commitments. It simply allows the government to finance existing legal obligations that Congresses and presidents of both parties have made in the past.” (www.treasury.gov/initiatives/Pages/debtlimit.aspx)

In a perfect world, you would be right: there would be no need for a separate debt limit over and above existing Congressional and Presidential oversight over U.S debt levels. In practice, however, a separate debt limit can be useful for two reasons. First, members of the Congress who are not directly involved with the congressional budget or appropriations committees can still influence spending and there needs to be some overall check on the system to prevent such members from spending too much. Second, there is considerable uncertainty associated with the projections surrounding spending and revenues and it is quite possible that federal borrowing may exceed revenues for some length of time. A debt limit is a form of fiscal accountability that requires Congress and the President to take “visible action” (to raise the limit) thereby attracting additional scrutiny to the nation’s finances. Think of the debt limit as an additional layer of protection against profligacy.