"Popular Opinion" is a catch phrase used by propagandists/public relations firms/psychiatrists that is designed to sway people's opinion into a desired direction, usually involving swaying the thoughts/opinions of the masses in the propagandists' favor.

By adding the word "popular" to the word "opinion" it makes most sheeple feel that they, too, want to be "popular" so their "opinion" will be the same as that opinion which is "popular."

In other words, people are easily molded and swayed into believing things that they normally wouldn't believe.