I would argue that race was not a particularly important factor. It is true that there were some attempts to use race to split the populist vote in the South. However, I think there were much more important factors.
First, just as today, populism does not tend to work on a large scale. Where there is a large middle class, that middle class tends to be much less supportive of populist points of view. Instead, they tend to be more supportive of big business.
Second, the big businesses tended to be very politically powerful in those days. They were able to use their financial power to “buy” the support of many political figures. They did not really need to use race to defeat the populists.
I would argue, then, that economic and class factors were more important than race.