Jeffrey Isaac, editor of Perspectives on Politics, has a thoughtful post in Duck of Minerva about not using the word "rejection."
The purpose of decision letters is not to say “yes” or “no.” It is to communicate honestly with every author in a way that is substantive and scholarly, and also collegial, constructive, and encouraging—which I take to be important scholarly values. And by communicating in such a way, we are fostering an intellectual community based on intellectual seriousness and mutual respect.
I'm undecided on this. Being polite and collegial is important, and so I can see how "decline" could be viewed as much more collegial than "reject." At The Latin Americanist we do use the "decline for publication." It would be interesting to go through a list of journals and catalog what language they use.
At the same time, I wonder how much this changes the relationship between editor and author. In other words, I am not sure how transformative it is for that relationship or for the discipline. Regardless of what you say, the person's paper will not get published in that journal, and they must now sort through discussions of why reviewers felt that way. Does "decline" make that an easier process?
I've submitted plenty of articles, and had plenty rejected/declined. Frankly, I don't remember much about the language of any of them. For fun and self-flagellation, I went back over a few in recent years. I see "decided not to accept." Another actually never said "decline" or "reject" but simply explained, finishing with "this is not a great fit." Another was "I am sorry to inform you that your manuscript will not be published." I tend to submit to interdisciplinary journals so I am not sure if straight political science journals are different.