This is certainly true in education and journalism. Two examples of the influence of the left in public schools I saw in my daughters' experiences are that the meaning of the Second Amendment was wrongly stated in a U.S. history textbook and recycling is part of what my school district calls a "Think Earth" curriculum.
Whether one is on the left or right, it cannot be denied that the left has had an enormous impact on the major institutions of American society — specifically journalism, education and the judiciary.
At the newspaper where I work, by and large most of the reporters are simply reporting the news, like city council meetings or features about locals who have made notable accomplishments. Once in a while though the politics creeps in. It depends on the beat. Our environment reporter gets annoyed at having to report the contrarian view of climate change and our health care reporter is decidedly in favor of a single-payer state-run system (she was actually a finalist to work as a consultant for a lawmaker crafting one of the major health care reform proposals). At least our growth and development writer isn't against property rights.
Out of a newsroom of about 50 people, two are Republicans. The rest are about 60/40 Democrat to nonpartisan, according to a spreadsheet of our voter rolls.
The kinds of people who get into the three crafts that Prager describes tend to be idealists, where producers or people who like to make things tend to go into business. Journalists of a conservative persuasion tend to go toward right-leaning publications where they can express their views. That leaves the mainstream media largely to liberals. The same could be said about teachers.