The parties really did not switch, certain party allegiances did. Black voters abandoned the Republican party when it became apparent that Reconstruction would be unable to continue. They joined the Democratic Party when FDR promised reform. The Constitutional interpretations of the two parties did switch as well, with the Federalist Party (the ideological Republican party) favoring a loose construction and the Democratic-Republican Party (the ideological Democratic Party) favoring a strict construction. Their goals never changed though.

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"It is extremely difficult for our contemporaries to conceive of the conditions of free banking because they take government interference with banking for granted and as necessary"

-- Ludwig von Mises