Zell is right, but take this to its logical conclusion.
Let's say that the dems see the light and tack right, allowing the free exchange of ideas from pro-life, pro-marriage (man-woman), strong defense and lower taxes. There are many in the dem party that are there only because of the vehicle, not because they are dems. They will look elsewhere and there will be a stronger green party, which will be even more radical but appeal to the disenfranchised dems. They cannot tolerate dissent to their socialist/radial cause.
The dems will shrink in power but be bigger than but not as enthusiastic as the extreme greens. They will become a permanent minority party.
The pubs have lots of RINOs who disagree with pro-life, pro-marriage (man-woman), strong defense and lower taxes. But the party accommodates them and they really have nowhere to go now. If the dems moderate, they may leave, but that is unlikely as long as the dems are a minority party, since they will lose their power. Plus, it is fun being an upstart and getting all the attention.
So if the dems follow Zell's advice, they will become a minority party for a long time, but if they do not they will remain a minority party for a long time. Catch 22.
For the sake of argument, it would mean our country had moved to the right.
I don't think Hillary can attract enough people to give her the WH in 2008. She will try to convince voters she's a centrist, which is laughable at best, but she is as far left as they go.