When former President Donald J. Trump questioned Vice President Kamala Harris’s racial identity in Chicago on Wednesday, he neglected a reality: that the country’s demographics have changed in recent decades, as more than 12 percent of Americans now identify as multiracial.

Beneath that fact is another, lesser-known shift. The number of Americans whose identities include being both Black and Asian has tripled over the past 15 years to more than 600,000, according to a New York Times analysis, a group that includes Ms. Harris, whose mother emigrated from India and father emigrated from Jamaica.

They are part of a jump in multiracial Americans that demographers have tracked for decades, a marked rise that reflects the steadily increasing diversity of the U.S. population. The numbers of Latino and Asian people have risen sharply since the 1990s, and so has the rate of marriage between people of different races. This has resulted in more multiracial children.

The U.S. Census Bureau also updated its methodology in 2020 and provided more ways for people to identify as multiracial, a change that demographers say better reflects the reality of the nation but also contributes to a rise in the figures.

In Mr. Trump’s comments on Wednesday, made to a roomful of Black journalists at a conference, he questioned whether Ms. Harris truly identified as two races.