As the monopoly issuer of the US dollar, the federal government doesn't need to tax its citizens or borrow money from itself or another nation like China in order to spend.  It can literally create new dollars out of thin air via keystrokes for a particular purpose as appropriated by Congress.

Thus unlike a private household or business or even a nation like Greece which gave up its monetary sovereignty when it joined the EU, the US federal government can never go broke.  Furthermore, federal deficits mean surpluses in the private sector, while federal surpluses mean deficits in the private sector which ultimately lead to recessions or worse depressions as the private sector is merely a currency user, not an issuer like the federal government.

The only natural constraint to federal deficit spending is inflation which federal taxes are used to keep under control by removing excess dollars out of the economy, thus giving our currency its value.  But they do not fund federal spending.