Benjamin Franklin, after leaving the Constitutional Convention was asked by someone what kind of government they had established. He replied, “A republic if you keep it.”
Most people who took a government class in middle or junior high school learned that the United States is a Constitutional Republic. I’m not sure if this information is still taught to students.
Here is what the Constitution says:
Article IV, Section 4. The United States guarantees to each State of this Union, a Republican form of government, and protects them from invasion; and, on application of the Legislature or the Executive, (when the Legislature is not convenable), against domestic violence.