I dunno about the biggest famine in history—it would take a lot of people dying to match the higher death toll estimates for the Great Chinese Famine of 1959-1961. I could see things getting pretty messed up, though: sharp rises in food prices because of lowered yields, the need for imports going up, profiteering on the international market, lower-income people being unable to buy enough calories to survive. Unfortunately, it's likely that the urban poor, rather than the rural white people who are the core of the Republican voting base, would be the ones disproportionaly affected.