"You are the first black president," reporter April Ryan said to Barack Obama at his final news conference on Wednesday. "Do you expect the country to see this again?"

"If in fact we continue to keep opportunity open to everybody," Obama replied, "then, yeah, we're going to have a woman president. We're going to have a Latino president. And we'll have a Jewish president, a Hindu president. You know, who knows who we're going to have. I suspect we'll have a whole bunch of mixed up presidents at some point that nobody really knows what to call them."

This is the American promise, really, that anyone of any background might ascend to the highest office in the freest nation in the world. In practice, of course, that hasn't really been the experience: Every president, save one, has been a white man, presumably (if not provably) Christian and straight. This year, a black man, two women, two Hispanic men and a Jewish man all competed for the presidency as candidates for a major-party nomination. The winner was another white man.

For decades, Gallup has tracked America's willingness to at least consider nonwhite, non-male contenders for the White House.

The first thing you'll notice is that willingness to back a member of a minority demographic tends to increase over time. In 1959, less than half of respondents to the Gallup survey said they'd back a black presidential candidate. By 1978, the figure was over 75 percent and, in the most recent poll from 2015, it hit 92 percent.

You'll also notice that different groups pop up in Gallup's polling as the prospect of electing them appears more real. "Socialist" was added in 2015 because of the candidacy of Bernie Sanders. Mormon cropped up in 1967 presumably thanks to the presidential candidacy of George Romney, Mitt's father. The number of people willing to consider a gay presidential candidate (later expanded to gay or lesbian) was near 25 percent in 1978 but tripled by 2015 -- passing the number who'd back a Muslim or an atheist.

What this suggests, then, is that familiarity with demographic groups as participants in American politics also increases the willingness of Americans to consider members of those groups as candidates. It suggests that Obama's prediction that the country would eventually see its way to electing members of each of these groups might, on a long enough timeline, come to fruition.

But of course being willing to vote for a member of a demographic group doesn't mean that members of that group can win an actual election. More than 90 percent of the country said they would be willing to vote for someone who is black, Hispanic, Jewish, Catholic or a woman. The country had a chance to elect a major-party candidate from each of those groups in 2016. It didn't -- popular vote notwithstanding.

The continued dominance of whites in politics (including on Capitol Hill) derives in part from the fact that white people (if not white men) are still the majority in the United States. Demographic groups tend to support members of their own group in politics. At least three-quarters of America was willing to support a black president for 30 years before the country actually elected one, only after the density of nonwhites in the electorate had increased to nearly 25 percent.

Demographic trends continue to shift and, perhaps within 50 years, more Americans will be nonwhite than white. But whites will still be the plurality for some time to come.

The more important part of Obama's comment to his point was how it began: "If in fact we continue to keep opportunity open to everybody." That's a statement about his desire to see America continue to embrace diversity. It's a statement that seems to act as a quiet rejoinder to the politics of the white man who will soon occupy the White House.