Imagine if a leader within the tea party movement were able to persuade its members to establish a third political party. Imagine he succeeded—overwhelmingly—and that as their leader he stood a real chance of winning the presidency. Then imagine that in anticipation of his electoral victory, the Democrats and Republicans quickly modified an existing antidiscrimination law so that he could be convicted for statements he made on the campaign trail.

All of this seems impossible in a 21st-century liberal...