Complex sentence: Some Populists believed that Jews made up a class of international financiers whose policies had ruined small family farms, they asserted, owned the banks and promoted the gold standard, the chief sources of their impoverishment.
Split into simpler sentences: Some Populists believed that Jews made up a class of international financiers whose policies had ruined small family farms. Jews, they asserted, owned the banks and promoted the gold standard, the chief sources of their impoverishment.