The diffusion argument held that spreading slavery to more territories would not only weaken the institution of slavery. But also that such a diffusion was necessary to the eventual abolition of slavery. Spreading slavery is necessary to end it? Freehling asks, "Is posterity supposed to believe that Southerners believed this stuff? That expanding slavery could best end it?! That saving the institution in Missouri could best eliminate it elsewhere?! These propositions the more strain credulity because they functioned so self-servingly. What a wonderful way to feel good about a supposedly evil way to make profits: expand the profits and you will end the evil!"
And he answers, "The diffusion argument, for some Southerners all the time and for all Southerners some of the time, operated as just this kind of noxious sedative." Yet he adds, "But to dismiss diffusion as entirely cynical or self-serving is to miss a revelation of the southern mentality. From posterity's perspective, as from [New York Republican Congressman] James Tallmadge's, stopping slavery from expanding seems the path towards ending it, while allowing slavery to spread seems the trail towards saving the institution."
As Freehling details in that book, the abolition of slavery happened in northern states as the white population grew and the black slave population declined. That not only meant the reduction of the power of slaveowners in relation to competing interests, like white fears of competition from enslaved labor, concerns that slavery would undermine the (white men's) democracy, and even genuine moral and religious opposition to slavery.
Tallmadge in 1819 had offered two amendments to the statehood bill for Missouri to restrict slavery. And his proposals built on the experience of what was known as gradual emancipation in New York, in particular:
In 1817, the congressman had helped secure New York's final emancipation act, freeing all slaves ten years hence. In 1819, the Tallmadge Amendments proposed a more limited abolition, reserved exclusively for slaves thereafter born in Missouri. Tallmadge's proposed age for freeing post-nati[vity] slaves in Missouri, 25, was exactly what New York had enacted in 1799 for post-nati black females (New York post-nati males had been declared freed at 28). In 1819, Missouri had about the same small number of blacks, around 10,000, as did New York. Missouri's relatively low slave percentage, around 16%, was about the same as New York's in the colonial period. The proposed new state of Missouri, not very enslaved or very black or very far south, invited a Yankee attempt to nudge southern apologists away from procrastination.This is an important point in understanding the antebellum politics of slavery. The disappearance of slavery was associated in real history with a reduction in the relative presence of blacks. And, conversely, the presence of black people was associated with the presence of slavery. There were certainly irrational elements in that sensibility. But political opinions are not always driven by careful analysis of sociological data, to put it mildly. (Not to mention that sociology as such didn't even exist in 1820.)
James Tallmadge proposed Thomas Jefferson's sort of gingerly nudge. No black born before 1820 would be freed. No slave born after the law passed need be freed before 1844, or indeed ever. Nor need a large Missouri free black population ever exist. Black ratios were low, no slave could enter in the future, and blacks could be sold down river before emancipating birthdays, as had been done in Tallmadge's state. [my emphasis]
It's also an important element in debunking neo-Confederate pseudohistory. One of the polemical arguments the pseudohistory makes is that the Civil War couldn't have been about ending slavery, because Yankee whites hated black people, too.
And it's true that most white Yankees had racist attitudes toward blacks, and in many cases it was virulent. That kind of hatred was on open display in violent protest and even murders against Abolitionists, and some Northern riots against free blacks. The most notorious of the latter was the anti-draft riot in New York in 1863 during the Civil War.
The implication of this argument is that white Americans in the antebellum period would only have opposed slavery if most of them embraced something that looked likely the Kennedy-Johnson liberal attitudes on race of the 1960s. That's a logical leap. But far more importantly, in the real history of the United States, hostility to slavery widely coexisted with hostility toward black people. The white Abolitionist John Brown's his egalitarian outlook on the full equality of whites and blacks was more of an outlier in the US of the 1850s than it was representative.