They cannot by default. It is illegal for them to default as long as there is money to pay the interest. Interest must be paid first before discretionary spending and they have more than enough to do so.
Interest: $568 billion per year Tax revenue: $4.6 billion per year
Even when you add in Medicare, SS, and Defense, they still have enough
Medicare: $1560 billion/year Social Security: $1280 billion/year Defense: $780 billion/year
All of this adds up to $4188 billion annually, which leaves only $412 billion for the rest of the federal government. This would shut down all non-essential government functions, which most of the rest of the country would see no ill effects, if not love. I say DO NOT raise the debt ceiling again.
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