Here are six answers to the question in my heading. Arguments can be made in favor of each.

1. The founding took place from 1776-1789, from the Declaration to the Constitution. Although the architects of the new republic sought to recycle some existing materials, they drew a new blueprint for the base on which our system stands. It was designed to be alterable–and it has been altered–but the foundation is still recognizable.
2. The foundation was poured in 1492 and 1619. Once Europeans began seizing land from indigenous people and importing enslaved Africans to work that land for them, the basic arrangement was set. The 1788 Constitution essentially preserved that structure. Some better things have been built on top of it, but the original floor is still down there, not far below the surface and determining what can be constructed above.
3. The social contract has been renegotiated at several key points: 1776 (Declaration), 1781 (Articles of Confederation), 1788-9 (Constitution), 1864-5 (post-Civil War amendments), 1938-45 (the Supreme Court reverses itself and allows the welfare state), 1954 (Brown v Board), and arguably again since the 1980s. These shifts are fairly fundamental and not well described by treating the 1788-9 contract as still foundational. The 21st-century political system is incompatible with the Framers’ plan, but that is because we have chosen to lay new foundations.
4. The United States was founded in 1788-9. In the 1900s, we ignored some of its basic principles, such as the list of enumerated powers, without explicitly and legitimately renegotiating them. The foundation is still in place, but we have built unsound structures on top of or beyond it. The Constitution is “in exile” (a phrase apparently more used by critics of this view than proponents of it).
5. The actual political-economic system in which we live is fundamentally based on publicly traded corporations, industrial production, organized labor, regulatory agencies, credentialed professions, public and private bureaucracies, mass media, mass schooling, securities markets, electronic networks, science (as a set of powerful institutions), databases of people and objects, and a permanent war machine. These elements are not envisioned in the US Constitution, which influences them somewhat but hardly determines them. The same basic structure is evident, for example, in Canada. The modern foundation has been poured one layer at a time, but if you had to pick a symbolic date for this option, it might be 1908, when the first Model T rolled off the assembly line.
6. There is no foundation. A society is not well understood as a base and superstructure or as a single game with basic rules. It’s a complex, emergent system best understood as whole series of overlapping and interacting institutions, each with rules of its own that affect the other institutions’ rules. All is flux.

I realize that most people don’t explicitly discuss this question, yet I think that today’s opposing ideological camps would each answer it differently. It could even serve as an ideological Rorschach Test.

See also: constitutional piety; the Citizens United decision and the inadequate sociology of the US Constitution; the role of political science in civic education; polycentricity: the case for a (very) mixed economy.

Everyone should read Politics Is for Power: How to Move Beyond Political Hobbyism, Take Action, and Make Real Change by my friend and colleague Eitan Hersh. It is a gentle and disarming critique of how many of us spend our time and energy as citizens. It comes with valuable suggestions for how to improve our impact. Read the whole thing, but for a teaser, see Eitan’s recent New York Times op-ed, “Listen Up, Liberals: You Aren’t Doing Politics Right” (subtitled “Politics is about getting power to enact an agenda. And the only way to do that is face-to-face organizing.”)

My response is to audit my own political activity to select work that meets these criteria: 1) The issues and problems are important. 2) I might be able to shift some people’s opinions or behavior by expressing my views to individuals who trust–or could trust–me. And 3) These people have–or could have–influence over decisions, e.g., by voting in an actually contested upcoming election, by changing their own organizations, by building new organizations, or in other ways.

Using those criteria, here are some possible foci for my own attention, ranked from most valuable (#1) to least worthy (#7). Your list will be different, because everyone has a unique set of assets and opportunities.

1. Advocate for changes in the state and local policies and the available materials for civic education in US schools. This is not the world’s most important issue. (It isn’t the earth’s climate.) But I have been paid to work on it for decades and have some comparative advantages in terms of credibility, information, access, and networks. Then again, I must be careful not to be satisfied with working on this issue, for which I am paid and assessed. I should also be engaged on other issues in my own time.
2. Advocate for affordable housing in Cambridge, MA, where I live. Dense and affordable housing in a city with excellent public schools would be good for the climate and for racial and economic justice. Housing is a salient and contested issue in our city. Most neighbors believe in affordable housing abstractly, but the proposed policies are deeply contested. I hold views on the matter, and if I invested my time, I might be able to make persuasive arguments to undecided voters within my own networks of trust, and expand those networks.
3. Form relationships and exchange ideas with people in one or more other countries, especially countries that do not get a huge amount of attention in the US and about which I might have some direct knowledge. Even though the geographical scale is large, people can increase the odds of peace and understanding through informal diplomacy and by educating their own fellow citizens back home.
4. Advocate for policies within Tufts, where I work. I do this every week while sitting in committee meetings or sending emails. Sometimes, the issues are significant. I have an increment of influence here. But I also face both practical and ethical limitations as a middle-manager. There are issues on which it is appropriate and important for me to advocate, and others that really aren’t in my domain. Drawing that line can be an ethical challenge for anyone who works within a Weberian organization.
5. Advocate specific policies to presidential primary candidates and legislators. I am not going to accomplish anything by taking a stand on the major policy issues of the day, such as single-payer healthcare or Iran. But there are specific issues on which I might have some special expertise and credibility and an ability to be mildly influential. For example, I believe that a Green New Deal (of any scale) must incorporate citizen participation in order to be effective. This is something I could advocate.
6. Take and express a view on the Democratic presidential primary candidates. Millions of others are also doing that, and almost every point that could be made has been made. Still, my social network includes a wide distribution of Democratic primary voters, from strong Democratic Socialists to committed centrists, and I suppose I might shift someone’s view by making a good point. (The reason I haven’t done this yet is that I am deeply torn and don’t know where I stand. Maybe I should just figure that out for myself and quietly cast my secret ballot on March 3.)
7. Take and express a position on the impeachment of Donald J. Trump. The decision-makers are the members of the US Senate. My senators (Warren and Markey) are 100% likely to convict. Approximately 1 bazillion words have already been said or written about this topic. Among those words are many annoying ones that I could criticize all day. But everyone I know has already made up their minds. I have no special expertise, influence, or leverage. This is one of those bright, shiny objects that lures my attention and distracts me from actually improving the world.