Unless evolutionary biology results in a fallen nature that may, on occasion, be overcome with the Grace of God.
The 'realist' view the author speaks of is a direct descendant of the Judaeo/Christian notion of the fall. It is a view of human nature that, if posed as a hypothesis, would have overwhelming historical support, just in the last century. It is not surprising that Burke, Locke and the founding fathers all grew up in overtly Christian nations. Their view of human nature was a Judaeo/Christian view, regardless whether it was explicitly grounded on scripture.
Fortunately, our nation was founded by folks with such a viewpoint and the consitution was designed for fallen man. That's the heart of the dispute about the 'living constitution.' Should we break and then remake the bones of our society to accomodate perfectable, new man?
Western world views are still stuck in the Locke vs Rousseau dispute. Until we come up with a pill that changes the nature of man, Rousseau's disciples will continue to inflict untold misery on their compatriots here on Earth.
Hobbes thought man was from the start and by nature uncivil, one against the other. Rousseau the opposite, as naturally Good.
The Christian view is holds man was from the start and by nature civil, for and with man and God, but that sin destroyed that perfection. The Christian view holds that Christ death and resurrection redeems this nature and promises a resurrection after death.
Regarding the nature of mankind, its moral perfectability is still a promise in the Christian view, but not without divine agency. You'd be right to say this is the view that is predominant for conservatives.
First, that isn't the doctrine as I understand it. Second, unless you're maintaining that some human ancestor species was in a state of grace, you can't speak of man having a "fallen nature."