Here is a taste of Waldstreicher's piece at The Atlantic:
The three-fifths clause, which states that three-fifths of “all other persons” (i.e. slaves) will be counted for both taxation and representation, was a major boon to the slave states. This is well known; it’s astounding to see Wilentz try to pooh-pooh it. No, it wasn’t counting five-fifths, but counting 60 percent of slaves added enormously to slave-state power in the formative years of the republic. By 1800, northern critics called this phenomenon “the slave power” and called for its repeal. With the aid of the second article of the Constitution, which numbered presidential electors by adding the number of representatives in the House to the number of senators, the three-fifths clause enabled the elections of plantation masters Jefferson in 1800 and Polk in 1844.
Just as importantly, the tax liability for three-fifths of the slaves turned out to mean nothing. Sure the federal government could pass a head tax, but it almost never did. It hardly could when the taxes had to emerge from the House, where the South was 60 percent overrepresented. So the South gained political power, without having to surrender much of anything in exchange.
Read the entire article.