America is between a mixed economy and Socialism, with the world's largest welfare system, and 'social safety net.' 3/4 of the massive Federal budget goes to social welfare programs. From 1930 to perhaps the early 1980s, the US was a mixed economy.
Every aspect of the US economy is completely under regulation and Federal control. The number of Federal regulations has doubled since 1975. There are now over 170,000 pages of Federal regulations in total, governing every aspect of ... well, everything. Obama's Administration alone added over 11,000 new regulations in just his first two years in office. America is the ultimate regulation state (aka the opposite of a low regulation free market).
America is heavily taxed, and has a total government system that eats 40% of the economy, and at $6.4 trillion is larger than the economy of Japan. Those facts alone disqualify the US as being Capitalistic.
It's quite easy to demonstrate that America is in fact not Capitalistic any longer. A very short historical comparison of the economic structures (spending, taxes, welfare, regulations, etc) of the US economy in, say, 1820, 1890, 1910, 1930, 1975, 2013 - more than demonstrates the progression from highly unregulated Capitalism toward Socialism.
Capitalism requires an unregulated free market based economy. America is almost as far away from that as you can get before becoming a full blown Socialist nation. Even worse, America's variation of Socialism is going to be Fascism, and is rapidly trending that direction.