Wattenberg's contention seems to be that he didn't leave the Democratic Party, but that the radicalized Democratic Party left him.
He is also very interested in defending the "neo-con" label, which, as his book jacket says, "is used frequently as an insult by those who fail to understand the concept."
Regardless of how you feel about "neo-cons," it is obvious that people who are keen on throwing the word around usually don't fully understand the term (this is similar to folks who call people "fascists," yet cannot define fascism). By definition, to be a neo-con, one would have had to have first been an intellectual and a Democrat. Many neo-cons were Jewish, but this is not, of course, a prerequisite.
And while neo-cons tend to believe in promoting liberty around the world, one does not become a neo-con simply by supporting foreign intervention.
Though small in number, neo-cons were a vitally important to the conservative movement's success in the 1970s and 80s, adding intellectual fire-power to Reagan's Revolution.
But just as the Christian conservatives (who, like the neo-cons, were late additions to the movement, partly motivated by the emergence of moral relativism and the rise of the counterculture) are sometimes resented by other wings of the conservative movement.
I look forward to fully reading Wattenberg's take on this topic ...