His fellow Georgia Democrat, Jimmy Carter, said that Zell was disloyal. Zell had the audacity to speak to the GOP convention, which to Jimmy overrides anything else, apparently.
This particular issue with Jimmy Carter highlights what happens a lot in politics. Rather than talk about what Zell Miller stands for, or the issues he speaks of, people like Jimmy Carter refuse to discuss that. Instead, they indict him for not being a good Democrat.
Because to Carter, being a good Democrat is all that matters.