Why, according to this section, should the Antifederalists be thought of as "co-founders of the Constitution along with the Federalists"?

It is mostly that they demanded that a bill of rights be applied to the Constitution soon after ratification, in exchange for their votes in favor of the ratification the Constitution.

Even though this contributed to the Bill of Rights being introduced in 1791, many deserve some credit for that very critical part of the Constitution.

c) legal battles last five weeks after the election due to the narrow victory in several states.