In the New York Times, Thomas B. Edsall states:
Regardless of the ultimate success of the Republican strategy, these trends guarantee that race and ethnicity will be dominant themes underlying the 2012 election, infusing the debate over deficits, taxes and government spending. In the 140 years from the end of Reconstruction to the present, no matter what the motives of those engaging in the debate, these divisive issues have worked to the advantage of economic elites and there is no reason to believe this will change.
The end of Reconstruction is one good date to choose for pinpointing a time when promoting racism worked for economic elites.
You can take your pick, actually. Here's for starters:
1676: Bacon's Rebellion. A thousand residents of the Virginia Colony rose up, including whites, "former indentured servants, poor whites, and slaves" led by planter Nathaniel Bacon. They rebelled against the policies of Governor Berkeley, who was friendly to the Native Americans.
According to Wikipedia, "Their alliance disturbed the ruling class, who responded by hardening the racial caste of slavery. While the farmers did not succeed in their goal of driving Native Americans from Virginia, the rebellion did result in Berkeley being recalled to England."
Many dates could be chosen that illustrate the efficacy of promoting racism in serving the interests of economic elites.
How to "harden the racial caste of slavery"? Make free colonists of African descent into slaves! Thousands of free citizens of African descent lived in the Colonies for a hundred years before Bacon's Rebellion.
Perhaps in the middle of the 17th century, if you were one of several thousand Africans living in Virginia you certainly knew that your children would be free -- you might have that expectation. To suddenly find themselves involved in lifelong servitude, and then to realize that in fact their children might inherit the same status, that was a terrible blow, that was a terrible transformation.- Peter Wood, historian
1619: Arrival of first Africans at Virginia Colony
They had been stolen from a Spanish slave ship, traded for food, and left in Virginia.