What were Federalist reasons for supporting ratification of the Constitution?
Federalists believed the Constitution provided just the right mix of power and limitations on power.
The first government of the US was a one-house legislature with no executive. It couldn't raise money, it relied on the states for military power, and was generally seen as ineffective and weak.
The US Constitution was written to remedy those weaknesses and provide the US with a better, more representative form of government. It tried to balance power between the small and large states by having a two house legislature. It tried to balance power between the central and state governments. And, it split power between three branches of government.
All of this elaborate 'check and balance' system was intended to strengthen the central government while making sure that there were always forces in place to keep it from becoming too powerful.
Federalists campaigned to support ratification because they believed the Constitution was the best way to balance these needs. Those opposed to the Constitution called themselves Democratic Republicans. The battle over the Constitution was fought, state by state.
And when critics of the Constitution were successful in persuading many Americans that it took too much power from ordinary people, the Federalists promised to write a Bill of Rights that would also secure certain rights to the people themselves. These became the first ten amendments to the US Constitution.