UNITED NATIONS — Britain has Theresa May. Chile has Michelle Bachelet. In the United States, Hillary Clinton this year became the first woman to clinch a major party nomination for president.

The United Nations, though, still seems to be a man’s world.

Never in the 70-year history of the world body has a woman been secretary general. And at the moment, it looks like that is not going to change.

Half the 12 people who initially entered the race for the post this year are women, many more than ever before. None has gained much support from members of the Security Council.

In four informal polls so far, one man has held the clear lead: António Guterres, a former prime minister of Portugal who until recently headed the United Nations refugee agency. The poll results show council members have flip-flopped on their No. 2 choice for the job – but they have consistently chosen a man.

The list of candidates is by no means closed, and other women could enter as early as next week. But advocates of gender equity both in and out of the United Nations are bewildered.

“Very disappointed,” is how the Colombian ambassador, María Emma Mejía Vélez, put it after the first poll in July.

Ms. Mejía had helped assemble a coalition of 60 countries to advocate for a woman for the post. In its public appeal a couple of months ago, the coalition pointed out that the United Nations in its many declarations and documents has repeatedly pledged to promote gender equality, and said it had “the responsibility to lead by example.”

“It surprised me that the members of Security Council didn’t find any of the six worthy of being first or second,” Ms. Mejía said after the third poll, in late August.

The one candidate who publicly described herself as a feminist — Vesna Pusic, the former foreign minister of Croatia — got so little support in the council’s polls that she was the first to drop out.

The final vote is expected in October, when the council will send its choice to the full membership of the General Assembly, where approval is a foregone conclusion. The decision will come down to the Security Council’s five veto-wielding permanent members, Russia and the United States in particular. Both are expected to choose someone who they perceive does not threaten their national interests.

The secrecy surrounding the council’s informal polls — there is no telling who voted for whom and why — makes it difficult to explain the poor performance of the women in the race. But there are several theories.

Two of the best-known candidates may have suffered because they are perceived as too close to either Moscow or Washington. Irina Bokova, a Bulgarian diplomat and the head of the United Nations Educational, Scientific and Cultural Organization, Unesco, is widely described as pro-Russian. Susana Malcorra, the Argentine foreign minister and for nearly four years the chief of staff to the current secretary general, Ban Ki-moon, is widely seen as pro-American.

Both deny association with any camp, and both have tried to curry favor with Washington and Moscow. That the selection comes amid a worsening United States-Russia divide further complicates matters.

The unpopularity of the other women is even more mysterious.

Neither the United States nor Russia have expressed a clear gender preference. The Russian ambassador, Vitaly I. Churkin, warned early on that men should not be discriminated against.

What may trump gender in the private bargaining is a candidate’s national origin and potential to challenge the authority of the permanent members of the Security Council.

Russia has insisted that it is now Eastern Europe’s opportunity, which many diplomats interpret as an Eastern European whom the Kremlin could countenance. The men who have second-place finishes in the last straw polls all fit that description: Danilo Turk of Slovenia, Miroslav Lajcak of Slovakia, and Vuk Jeremic of Serbia. None are well known outside the region.

Which leads to conjecture over why the women seeking the post have fared poorly. Are the highest-profile ones — Ms. Malcorra, Ms. Bokova and Helen Clark, a former prime minister of New Zealand who leads the United Nations Development Program — being judged by a different yardstick?

“It is a bad sign that these three very accomplished women who have held high-level positions in the U.N. and know the U.N. very well are being judged by a double standard,” Charlotte Bunch, an academic and a longtime advocate for gender equality at the United Nations, wrote by email.

“They are as well or better prepared than the male candidates,” she said, adding that their qualifications also surpassed several previous secretaries general.

The executive director of the United Nations gender equality agency, known as U.N. Women, Phumzile Mlambo-Ngcuka, said there were “superbly qualified women” running for the job. “So it is both surprising and disappointing,” she said, “to find a glass ceiling apparently still in force.”

In 2015, the bulk of the secretary general’s senior appointments were men, according to an analysis by a former United Nations diplomat, Karin Landgren.

Ms. Mlambo-Ngcuka said that the next secretary general, whether a woman or a man, should make “a fundamental, feminist commitment to breaking the barriers to gender equality across the world.” That should include gender equity in staff positions, “zero impunity” for United Nations staff and peacekeepers accused of sexual abuse, and a refusal to take part in all-male panels, she wrote.

Mr. Guterres, the leading candidate, has repeatedly promised to promote gender equity, including appointing 50 percent of the top jobs to women.

The United Nations set that goal for itself 20 years ago, and is nowhere close to meeting it.