It would be interesting to find some Republican examples of this.

Yeah, to be perfectly fair, I can't think of any. There are some areas
where Republicans hold a similar dominance. These dynasties typically
haven't been running for as long as the Democrats have enjoyed their
uninterupted reigns. But that's only because the Democrats tend to be
firmly entrenched in older, eastern cities while the Republicans dominate in
younger, western ones. My real point, I guess, is that without any
competitions, local parties change into something that doesn't really match
up with national perception of the party. Even right wingers join the
Democratic party here and in cities like Chicago, because that's the only
way to vote in the "real" election -- the primary races for who wins the
Democratic nomination. The general election in November is just for show.
The real issues and decisions get made during the May primaries.

As a result, things get all mixed up. We actually have a few Democrats here
who occasionally argue (unsucessfully) against their more traditional
brethren for tax cuts, or all things. Boy, did that take some getting used
to when we first moved into town. It was like watching water flow uphill!