The truth is the federal government needn't rely on any funding at all to operate. It creates currency and backs it so it can simply create whatever money it needs to operate. The purpose of taxation on the federal level is simply to regulate inflation and to create demand for government issued currency. Federal taxation is completely unnecessary outside of those two goals, which can be served in other ways quite easily.
On the other hand local and state governments, which do require funding, can fund themselves quite easily with usage fees such as "property tax", lottery sales, licensing fees, various investments etc.
So really the whole thing is open to debate.