Dick Greenhaus: According to Wikipedia it was actually 1787, during the Constitutional Convention in Philadelphia PA. The rule is more popularly known as the Three-Fifths Compromise.

I'm sure that the slaveholders considered it "disenfranchising" to have lost those extra votes, since they were being denied voting rights that had been given to them in the Constitution that their states had ratified in good faith many decades earlier. Hence the lyrics in some of the political-campaign songs and the war songs about anti-abolitionists being the defenders of the Constitution, while figures like Lincoln are called tyrants (probably in an attempt to hearken back to the Revolutionary War era when George III was the tyrant and the future framers of the Constitution were the good guys).

To those in the "free states", of course, it was worse than irksome that Southern slaveholders wielded so much political power through those extra votes (and that, to the Northern mind, the slaveholders were the ones doing the "disenfranchising"... or so said the political propaganda, anyway).