The Anti-federalists opposed the ratification of the Constitution because they believed the new constitution would give too much power to the federal government.
Anti-federalists were concerned that the new US Constitution would result in a similar political structure to what the Americans experienced underneath the control of Great Britain. Considering that this government structure was a main cause in America declaring their independence, the Anti-federalists wanted to do everything they could from having any government similar to the one they had under Britain.