The President elected two years ago to revive American prosperity is now presiding over an economic boom. The first two years of the Trump era have been so good that his predecessor has lately been barnstorming the country trying to persuade voters that we’re still living in the Obama era. It is the sincerest form of political flattery. But as good as the economy is, rising optimism among voters is not just about the money.

Left-leaning economists continue to downplay the success of the Trump economic program and say that...