It's not just NATO, it's membership in any international organization of nations regardless of their purpose, NATO, UN, WHO, IMF, Paris Climate Agreement, TPP, OAS, etc. It's based on the mistaken belief that America's problems all come from abroad with little benefit, so isolating ourselves will solve our problems. This believe ignores the fact that 1/4 of American Citizens are 1st or 2nd generation immigrants, that exports are responsible for 10 million American jobs, foreign tourist add over a trillion dollars a year to the US economy , Americans have 6 trillion dollars in foreign investments and 45% of the sales from America's largest businesses come from abroad.
The fact is the US can not separate itself from global affairs.