I am not sure exactly what they mean by "prohibitively." I guess they mean that the "liberalness" of the electorate would prohibit those with other political ideas from running for office.This has led to a widespread expectation of an increasingly – and prohibitively – “liberal” American electorate.
The coalition of voters who elected Obama was thought to usher in a new era where progressive, leftist ideas (what in America is mis-called "liberalism") would rule triumphantly for a long, long time. The young and the emerging minority majorities (i.e. "people of color") would ensure electoral success for "liberal" Democrats for the foreseeable future. Books were written about this new electorate and how the Democrats would be in charge for at least 40 years.
So, people were expecting the electorate to be "liberal" and to grow more and more "liberal" over the years.