By Eugene Robinson

WASHINGTON — Let me get this straight: Kamala Harris and Julián Castro are out of the presidential race, while Pete Buttigieg and Tom Steyer are still in? And there may not be a single person of color on stage at the last debate before actual voting begins with the Iowa caucuses? And this is the Democratic Party we’re talking about?

Harris, who ended her campaign last month, is the first African American elected to the U.S. Senate from California, a state more populous than most nations, including Canada and Australia. Castro, who dropped out of the race Thursday, is a former secretary of housing and urban development and perhaps the best-known Latino politician in the country.

Buttigieg, by contrast, is the now-former mayor of the fourth-largest city in Indiana. And Steyer is a hedge-fund billionaire who has never held elective office. The fact that both happen to be white men is, well, a simple fact.

Campaigns generally end because they run out of money. Candidates fail to garner support and donations for a variety of reasons. But I seem to recall that last summer, when the debates began, Democratic Party pooh-bahs congratulated themselves on how the rainbow array of candidates reflected the racial and ethnic diversity of the party’s base. The debate scheduled for Jan. 14 in Des Moines, however, promises to be an all-white affair.

So far, the only candidates to qualify under the party’s arcane rules are former Vice President Joe Biden; Sen. Bernie Sanders, I-Vt.; Sen. Elizabeth Warren, D-Mass.; Sen. Amy Klobuchar, D-Minn.; and Buttigieg.

Active candidates who do not yet meet the standard — at least 225,000 unique donors; plus support of 5% or more in four party-approved national or early-state polls, or 7% or more in two party-approved early-state polls — include Cory Booker, the first African American senator from New Jersey, and businessman Andrew Yang, who is Asian American.

Steyer is almost certain to miss the debate, too, as is former New York Mayor Mike Bloomberg, another white man. But the shrinking field’s two billionaires have the resources to stay in the race as long as they want, blanketing the airwaves with campaign ads. Whether all that spending will translate into votes remains to be seen.

The Democratic Party should nominate the best possible candidate, regardless of race, and electing any of the above would be far, far better for the nation and the world than enduring four more years of President Trump’s incompetence and corruption. But with polls showing many Democrats are still shopping, the debate rules threaten not to aid the decision-making process but to distort it.

You will recall that the race began with well over 20 candidates, forcing the party to hold two-night debates. The rules were designed to winnow the field, and they did the job — perhaps all too well. Diversity is among the collateral damage.

Look at the situation Castro was facing. He lagged in the polls and was running low on funds.

Realistically, his only chance would have been to make an unexpectedly good showing in the Iowa caucuses — something that has revived near-dead campaigns in the past. But without being on stage in Des Moines for the last debate before caucus night, and lacking the money for some kind of big media blitz, he had no real chance of pulling off an Iowa surprise. So he’s out.

Look at Booker, who performed well in his last two debates and saw his fundraising pick up. Look at Yang, who has done surprisingly well thus far but likely won’t clear the Jan. 14 debate threshold. They, too, need to outperform expectations in the caucuses to remain viable as candidates. They, too, will suffer greatly from not being on that stage.

How many candidates ideally should be at the Des Moines debate? Certainly fewer than the 14 who are still actively running. But more than five.

Biden, Sanders, Warren, Buttigieg and Klobuchar have worked hard to get where they are. But Biden is a relatively weak front-runner, at this point, and nobody else registers even 20% in the Real Clear Politics average of national polls.

If ever there was the opportunity for someone back in the pack to vault to prominence with an out-of-nowhere performance in Iowa, this would seem to be the year.

Order had to be imposed on an unruly race, but the party’s debate rules have done so prematurely. As a result, the Democrats’ greatest electoral strength — diversity — likely will not be on display. “Whites only” is not a look the party should want.

Email Eugene Robinson at eugenerobinson@washpost.com