The likely answer is that Trump would be heading into 2020 as a colossus. With an unemployment rate at just 3.5 percent, he would have a wide and possibly leisurely path to reelection, rather than the narrow and arduous one he is facing. More than that, he likely would be basking in recognition of how many of his signature ideas have crashed through old barriers and are driving the agendas of both parties.

Trump’s ideas on trade — as reflected in this week’s deal on a new North American trade agreement — show how he has routed a free-trade orthodoxy that was once embraced by business-minded elites in both parties. Likewise, his view that the United States should recognize China as a long-term adversary on economics and projection of global power is ascendant in both parties. So, too, is his view on the need to extract the nation from “endless wars.” So, too, are his tolerant views on Big Government spending and his blithe rejection of old notions of fiscal restraint and discipline. His ideas on immigration, by contrast, are deeply polarizing, but even there he has drawn support from many working-class voters who used to be instinctual Democrats.