Sen. Kamala D. Harris’s (D-Calif.) sudden departure from the Democratic presidential primary contest on Tuesday marks a significant moment in the evolution of the field. The most diverse primary field in U.S. history is now one that may well be represented in its December debate by four white men and two white women.

The field has admittedly been very fluid, meaning that predictions about what will happen in a few weeks — much less two months — should be considered risky. But while most of those who’ve dropped out over the course of the year were white men (including two already this week), Harris was both the best-polling nonwhite candidate and, for a brief moment this summer, someone who looked as if she might ascend to the top tier of the crowded field. That surge, following the first debate, soon faded, and Harris never regained her momentum.

There have been 27 major or major-adjacent candidates who’ve declared at some point. Fifteen of them remain, including the recent additions of former New York mayor Mike Bloomberg and former Massachusetts governor Deval Patrick.

The debate criteria introduced by the Democratic Party have been effective at winnowing the field. While many of those still in the race haven’t been making the cut for recent debates, many of those who dropped out did so after failing to earn a debate slot.

But compare where we are now with what the field looked like on Valentine’s Day. At that point, most of those who had declared were women, black or Hispanic. Most of them are still in the race, even with Harris’s departure.

Now compare that with what the field looks like after Harris left.

Pay particular attention to that December debate column. It shows the six candidates who’ve earned a slot, all of whom are white. Harris had earned a slot but obviously won’t be participating following her departure from the race.

Again, this was by design. The party didn’t want to have two dozen candidates crowding the debate stage with only two months to go until voting began. Its strict criteria, though, have had the unexpected effect of making it easier for an extremely rich white man with no experience in elected office (Tom Steyer) to gin up the contributions and poll numbers needed to get a spot onstage, while experienced officials with more limited resources — like Sen. Cory Booker (D-N.J.) or former HUD secretary Julián Castro — appear to come up short. The three are all doing about as well in the polls as each other, but Steyer’s incessant ads asking for contributions or boosting him just enough in just the right places give him the needed edge.

It’s important to recognize, though, that this assessment of the collapse of diversity in the field overlooks two important factors.

The first is that we’re really only talking about racial and ethnic diversity. Sens. Elizabeth Warren (D-Mass.) and Amy Klobuchar (D-Minn.) will appear in December, as will candidates who would become the first Jewish (Vermont Sen. Bernie Sanders) or openly gay (South Bend, Ind., Mayor Pete Buttigieg) presidents in U.S. history.

The second is that one reason that the field is narrowing to mostly white candidates is Democratic voters overwhelmingly support white candidates. The Post’s most recent primary poll, conducted with ABC News, showed that white candidates earned more than 80 percent of all support from Democratic primary voters, driven by the two-thirds eaten up by former vice president Joe Biden, Warren and Sanders. White men got just under 60 percent of all support.

Among nonwhite voters, white men got more support than among Democratic voters overall, albeit so slightly that the difference isn’t statistically significant. Even that is noteworthy, though: White and nonwhite voters support white and nonwhite candidates about equally.

At least when considered in demographic groups. Biden fares much better with black primary voters than do other candidates, one central reason he continues to lead the field.

As CNN’s Ronald Brownstein has noted, no candidate has been able to cobble together a coalition like the one enjoyed by Barack Obama in 2008. Biden’s doing well with black voters, Warren and Buttigieg are jostling for college-educated whites, and Sanders is performing well with younger voters. A broad split — but, again, all white.

As is often the case in American politics in 2019, President Trump plays a role here. Defeating Trump continues to be a primary motivation for Democratic primary voters, and white Democratic candidates continue to be seen — however justifiably — as the candidates with the best chance of winning in 2020. In our November poll, 77 percent of black voters viewed Biden, Warren or Sanders as the candidates most capable of beating Trump.

Perceptions of electability can change quickly. It did for Harris after her first debate performance, and it has for other candidates in the past. So far this year, though, Biden’s edge on electability has been consistent, even as electability has become a central concern of voters. That has been an ongoing problem for his opponents.

The other problem, of course, is that you have to be a candidate to benefit from a sudden surge in views of your electability. With a field winnowing down to largely white candidates, the odds that the Democratic nominee isn’t white necessarily drop significantly.