Two Conceptions

 It seems to me, that while few people think explicitly in these terms, their underlying conceptions of what government is for, boil down to two categories:

People have certain rights, which are fundamental.  Nobody granted these rights, they stem from being Human.  Governments are formed by the people, primarily for the purpose of protecting these rights.  This first conceptualization is well represented in the Declaration of Independence.

We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness. That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed,

The  second conception of government is that the most fundamental rights are material in nature, not freedoms so much as food and shelter.  Governments are created to ensure everybody gets all the essentials of life.  The best exemplar of this is found in The Communist Manifesto. 

Question 2: What is the aim of the Communists? Answer: To organise society in such a way that every member of it can develop and use all his capabilities and powers in complete freedom and without thereby infringing the basic conditions of this society. 

Question 3: How do you wish to achieve this aim? Answer: By the elimination of private property and its replacement by community of property

I believe that both are based upon opposite conclusions regarding opportunity:  

For Marx, the assumption is that society is stratified:  If you are born poor, you will remain poor.  The owners of farms, factories and so forth will only pay you enough for you to live, but never to accumulate capital of your own. 

For our founders, the assumption was opposite:  They certainly acknowledged that the rich have advantages, but they believed that, at least here in the United States, a person could accumulate wealth, no matter their starting point.

Who was right?  To some degree both: There are those, who because of their starting point, talent and initiative may find it impossible to advance to the top. On the other hand, most people can, through diligence and effort, better themselves.

Some evidence:

There is a lot of mobility in the US, both up and down.