Imagine two decades ago, or even one, that the 37-year-old, openly gay mayor of a medium-size American city appeared on stage and declared he was running for president. He either would have been laughed off that stage, or widely ignored.

Today, that person, South Bend, Ind., Mayor Pete Buttigieg, isn’t simply making a run, he may be the hottest candidate in the land. He raised more than $7 million in the first quarter of the year, has qualified for Democratic primary debates, has moved the needle with policy proposals and is getting wide media attention, including an appearance on “Meet the Press” Sunday.

Much of this surprising evolution comes because of Mr. Buttigieg’s own intelligence and considerable political skill set, of course. But his candidacy also is both a sure sign of, and a result of, the destruction of the old political patterns. The long-prevailing rules of politics are shattered, the dominance of the traditional political establishment has waned, and there are no clear rules about what comes next.

The Buttigieg candidacy, in short, is a sign that the political system is at an inflection point, and voters are prepared to think outside the box. “I’ve bet a lot on that premise,” Mr. Buttigieg said in an interview.

Put another way, as different as the two men are in most every way, Candidate Buttigieg might not exist without the example of President Trump, who shattered expectations and all the old paradigms in 2016.