David Brown in Attacking Slavery from Within: The Making of "The Impending Crisis of the South" Journal of Southern History 70:3 (Aug 2004) explains that Helper had seen some of the baleful effects of the slave economy on his native piedmont section in North Carolina:
Moreover, economic trends within the central piedmont gradually worsened the position of nonslaveholders during the 1850s. Charles C. Bolton calculates that landless poor whites, who were mainly tenant farmers and laborers, composed somewhere between 30 and 40 percent of all free white households in the central piedmont in 1860, and he suggests that slavery restricted their economic opportunities. Most significantly, the chance of owning land and joining the ranks of the yeoman farmer diminished in the 1850s as the presence of slaves both lessened the need for and drove down the price of casual wage labor. Such work, routinely required by the agricultural economy, was one way that poor whites might eventually accumulate enough savings to become upwardly mobile. Other potential avenues of employment, such as working in the mining or turpentine industries, were also subject to competition from slave labor. The move toward more commercial farming, as railroads made the piedmont accessible to distant markets, also harmed the position of poor whites because the net effect was to raise land prices. Bolton concludes that there was a permanent and growing class of landless whites in the central piedmont whose only option was to leave the area, as they did in increasing numbers during the 1850s. Notably, Helper included in The Impending Crisis Benjamin Hedrick' s lament about the plight of families forced to leave the region. This was the economic situation of the piedmont in the mid- l 850s, with which Helper would have been personally familiar. We know that he placed great faith in the importance of statistics and quoted extensively from the 1850 census. However, he was also surely influenced by his own observations of greater numbers of landless whites, more frequent departures of friends and family, and a general sense of restricted economic opportunity in the piedmont. This situation stood in stark contrast to his impressions of the bustling urban North. Helper would have encountered a different economic scenario had he been resident elsewhere in the South and, accordingly, would have written a different book. [my emphasis]Brown points out Helper expressed moral objections to slavery. But he goes so far as to argue that Hinton Rowan Helper was not motivated [in his antislavery] by racial hatred or by
personal envy and most importantly was not an outsider, but instead he genuinely believed that he held the best interests of the South at heart. He attacked slavery from within."
As we've seen, hostility to slavery among whites often went hand-in-hand with hostility to blacks. Helper is usually taken to have been an example of that. Brown in this article questions the contemporary evidence for anti-black racism in Helper in the 1850s. He notes that Helper was specifically dubious about the (pseudo-) "scientific racism" that was so popular among slaveholders. Brown doesn't argue that Helper was free of antiblack racism in the 1850s. His point is that among the motivations for writing his Impending Crisis, "antiblack prejudice was not a prime, or even a minor, motivation."
As Brown notes, Helper's postwar books were distinctly racist. As the trusty Britannica Online's article on Hinton Rowan Helper, "After the war, he wrote three bitter racist tracts advocating deportation of blacks to Africa or Latin America."
The full text of Impending Crisis is available at the University of North Carolina's Documenting the American South website.