But in 2020, Mr. Trump and Joseph R. Biden Jr. face a drastically changed electorate. The cohort of non-college-educated white voters — who gave Mr. Trump just enough of a margin to win the election in 2016 — has been in a long-term decline, while both minority voters and white college-educated voters have steadily increased.

The decline, a demographic glacier driven largely by aging, has continued since 2016. The number of voting-age white Americans without college degrees has dropped by more than five million in the past four years, while the number of minority voters and college-educated white voters has collectively increased by more than 13 million in the same period. In key swing states, the changes far outstrip Mr. Trump’s narrow 2016 margins.

His campaign leaders are betting that a two-year grass-roots mobilization that has yielded significant voter registration gains will overcome the demographic disadvantage and the polls, again.