They seek an emotionally satisfying outlet for their fury at their country’s decline, the decade-long collapse in the credibility of its institutions, and the rapidly changing demographic makeup of its leadership.
Scherer does not say, and I'm sure does not mean, that "the rapidly changing demographic makeup" of U.S. leadership is causally connected to the country's alleged decline and the "collapsed credibility of its institutions." But grouping the three together as comma-separated items does seem to imply a causal link, or an association of diversity with decline.
In any case, all three alleged trends are questionable. As James Fallows has written, reports of American decline have been rampant since Sputnik (and doubtless much earlier). Different institutions have lost (and gained) different degrees of credibility at different times (Congress lost crediblity, in my book, with the advent of the Gingrich crew; the Bill of Rights lost credibility with the Bush torture regime; most branches of the executive branch have doubtless gained significant credibility since Obama's inauguration; the military is far superior to that of the Vietnam era). And while Obama himself is black (or half black), and doubtless the number of people of different ethnicities in various leadership positions continues to increase, this process has been going on for some time.