To make a limited point...
Something I noticed. I have spent years, actually decades, bearing attacks on candidates and intellectuals who are perceived by the media as "right wing." And so, I have heard Barry Goldwater characterized and visualized repeatedly as a "fascist," a "Nazi." And I have heard Ronald Reagan identified as a fascist and Ayn Rand repeatedly call a "fascist." Now, I see and hear Donald Trump identified again and again as a "fascist," an aspiring Mussolini, and so on. These attacks on trump by, for example, Jeffrey MacDoald, are cast in the same terms and I bristle. so let us keep in mind, firmly, that a dissenter from the Liberal-Progressive Establishment in not automtically "fascist" or absolutely obviously and automatically "racist." Those extreme terms of attack must be demontrated...
Some on the right retaliate by using the terms "socialist", "Progressive" or occasionaly "Commie". Using this language doesn't solve the problem, it makes it worse.
However, the right is much less often guilty of this. The left, in particular the Progress branch of the left and Alinski-ites" and use his techniques to the fullest estent possible. Look at Obama and Clinton. both were raise on Alinsky's teachings and methods and have used the to great effect. Demonize, marginalize and delegitimize. All start with the tagging of people as something undesirable or worthy of contempt.
What is the reason it is becoming more and more common? It works. Whenapplied long enough, consistently enough and brutally enough, it works. That is why negative adds work. No one likes thiem but they work.