Keep in mind that Lieberman was an attempt to make a play for Florida (by increasing turnout in the fairly significant Florida Jewish community in particular). Indeed, he was probably a net positive (his selection generated some of the only significant positive media coverage Gore experienced all campaign). Most of the issues that caused Lieberman to become unpopular didn't really become prominent until after 9/11; in 2000 Lieberman was best known as generic New England Democratic Senator who also happened to be the first Jewish candidate for VPOTUS.
So a different candidate could easily end up worse for Gore.
and while we're at it, I was disappointed that Gore endorsed Dean in the '04 Democratic primary.
I mean, he selects Lieberman as his vice-presidential nominee and the person potentially a heartbeat away from the presidency, and then he can't find a way to support him four years later in the primary? ?
Or, follow your mother's advice that if you can't say anything nice, don't say anything at all. Al could have just stayed on the sidelines.