Why did James Madison refer to the legislative branch as the first branch of government?

Madison
answers this question most directly in Federalist No. 51, the essay where he addresses the
system of checks and balances built into the Constitution. 

Madison tells his
readers that in "republican government, the legislative authority necessarily
predominates." This is because in this kind of government "a dependence on the people
is...the primary control on the government," and obviously the legislative branch is most
dependent on the people. This Congress under the Constitution has the power to tax, to declare
war, to borrow money, and to carry out almost all of the functions of a government. The Congress
can also impeach and remove members of the other branches for a number of offenses, and the
Senate has the power to give, or not give, consent to both treaties and presidential
nominees.

While the Framers of the Constitution clearly wished to create a
far more powerful central government than had existed under the Articles of Confederation (which
only had a legislative...

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