President Trump’s 2020 election strategy relies largely on the white, working-class base that he excited in 2016. But he faces a demographic challenge: The electorate has changed since he was last on the ballot in ways likely to benefit Democrats.

Working-class, white voters are projected to decline by 2.3 percentage points nationally as a share of eligible voters, compared with the last election, because they are older and therefore dying at a faster rate than are Democratic groups. As those voters pass on, they are most...