So, what about the Constitution made people in the late 1780s (the "Critical Period") nervous or fearful of the new federal government that was being proposed?

When the Constitution was approved in 1787, a large part of American society fell into fear of the political and administrative system it created for the United States.

Thus, the Constitution established a presidential regime in which the executive power would be exercised by a unipersonal governing body, the President, which would be limited in its powers by the Congress and the Supreme Court based on checks and balances.

Even so, many people (the so-called anti-federalists) believed that this type of one-man government could degenerate into an absolutist and totalitarian system, just like what was previously suffered at the hands of the British Crown.