Saying his ideas are well-supported by history is a bit generous considering his historical arguments always seem to boil down to a barrage of anecdotal evidence of dubious factual value. To be fair, I've only read the Underground History and not Dumbing us Down, but here's a quote from wikipedia:
"The publicity generated by the initial success and compounded by the publishing disagreements propelled the pamphlet to incredible sales and circulation. Following Paine's own estimate of the pamphlet's sales, some historians claim that Common Sense sold almost 100,000 copies in 1776,[13] and according to Paine, 120,000 copies were sold in the first three months. One biographer estimates that 500,000 copies sold in the first year (in both America and Europe, predominantly France and Britain), and another writes that Paine's pamphlet went through 25 published editions in the first year alone.[7][14] However, some historians dispute these figures as implausible because of the literate population at the time and estimated the far upper limit as 75,000 copies."
Now, this gets into a bit of a weird circular argument where the book didn't sell as many copies because the people weren't literate and therefore the people weren't literate because the book didn't sell so many copies. Still, claiming that the book sold 600,000 copies seems disingenuous unless Gatto expanded upon it. I don't remember him citing his sources either in his book, but it's not like I've looked at the sources either, so whatever.
"The publicity generated by the initial success and compounded by the publishing disagreements propelled the pamphlet to incredible sales and circulation. Following Paine's own estimate of the pamphlet's sales, some historians claim that Common Sense sold almost 100,000 copies in 1776,[13] and according to Paine, 120,000 copies were sold in the first three months. One biographer estimates that 500,000 copies sold in the first year (in both America and Europe, predominantly France and Britain), and another writes that Paine's pamphlet went through 25 published editions in the first year alone.[7][14] However, some historians dispute these figures as implausible because of the literate population at the time and estimated the far upper limit as 75,000 copies."
Now, this gets into a bit of a weird circular argument where the book didn't sell as many copies because the people weren't literate and therefore the people weren't literate because the book didn't sell so many copies. Still, claiming that the book sold 600,000 copies seems disingenuous unless Gatto expanded upon it. I don't remember him citing his sources either in his book, but it's not like I've looked at the sources either, so whatever.