Many alliance members would like his succession settled at, or even before, a NATO summit in Lithuania in mid-July.

 

That does not give NATO's 31 nations, spanning from the United States through new member Finland to Türkiye, much time to forge the consensus needed to pick a new leader. They could also ask Stoltenberg to extend his term for a fourth time.

 

Whoever takes the reins will do so at a critical time, facing the twin challenges of keeping allies together in supporting Ukraine while guarding against any escalation that would draw NATO directly into a war with Russia.

 

British Defence Secretary Ben Wallace declared last week that he would like the job. But, as some governments push for a first female NATO secretary-general, Danish Prime Minister Mette Frederiksen is also emerging as a serious contender.

 

Though a very public role, the contest is extremely opaque, played out mainly in consultations among leaders and diplomats. Those consultations continue until all NATO members agree they have reached a consensus.

 

Jamie Shea, a former senior NATO official who served with the alliance for 38 years, said leaders will be looking for a highly skilled politician, communicator and diplomat.

 

"Keeping the family together, keeping everybody on board constantly, being in touch with all of the allies to make sure that you're addressing their concerns, is an important part of the job," said Shea, now with the Chatham House think tank.

 

Many diplomats see Wallace as a long shot for the job, although he is widely respected across the alliance. The desire to pick a woman counts against him with some members.

 

Many would also prefer a former prime minister or president to ensure NATO's boss has top-level political clout. Stoltenberg, 64, was prime minister of Norway.

 

And some, notably France, want someone from a European Union country, hoping for closer cooperation between NATO and the EU.

 

Frederiksen meets all of the above criteria. Although she says she is not a candidate, she has stopped short of saying she is not interested in the job. NATO diplomats say that behind the scenes she is being seriously considered.

 

Frederiksen's name first surfaced publicly in a report by Norwegian newspaper VG last month and there was a flurry of media interest again this week when the White House announced she would visit U.S. President Joe Biden in early June.

 

"I'm not applying for any job postings," she told reporters in Copenhagen on Wednesday, playing down speculation that the visit might amount to a job interview for the NATO role.

 

While the post traditionally goes to a European, any serious candidate needs buy-in from Washington, NATO's dominant power.

 

A source familiar with U.S. thinking said the Biden administration does not yet have a favoured candidate and a "lively debate" among top aides was ongoing.

 

A State Department spokesperson said it was "too early in the process to speculate on who the United States will support".

 

Social Democrat Frederiksen, 45, became Denmark's youngest prime minister in 2019. She was praised for crisis management in the COVID-19 pandemic and won a second term last year.

 

She would have to give up her post as prime minister if she got the NATO job, which political commentators have said would bring her fragile government to the brink of collapse.

 

And a campaign for the NATO post would not be plain sailing.

 

Her country falls well short of the NATO goal of spending 2% of GDP on defence. Denmark is at 1.38%, although Frederiksen has pledged to speed up efforts to hit the target.

 

Some allies also argue the job should go to an Eastern European for the first time, particularly as Russia's war in Ukraine has made that region even more important for NATO.

 

If Frederiksen got the job, she would be the third NATO boss in a row from a Nordic country.

 

Estonian Prime Minister Kaja Kallas, European Commission President Ursula von der Leyen of Germany and Canadian Deputy Prime Minister Chrystia Freeland have also featured in discussions among diplomats and media reports.

 

But diplomats say Kallas is seen as too hawkish on Russia for some NATO members, Berlin wants von der Leyen to stay at the Commission, and Freeland faces major headwinds as a non-European from a country considered a laggard on defence spending.

 

Other names that often come up are veteran Dutch Prime Minister Mark Rutte and Spanish Prime Minister Pedro Sanchez.

 

But Rutte has insisted he does not want the job. And Sanchez has a general election to fight later this year.

 

Some diplomats also suspect many mooted candidates may be unacceptable to Turkish President Tayyip Erdogan, who is widely expected to be re-elected on Sunday and has shown no qualms about blocking NATO consensus.

 

Turkey, along with Hungary, is holding up Swedish membership of NATO.

 

The apparent shortage of candidates with widespread support raises the possibility of Stoltenberg's tenure being extended again, perhaps until another NATO summit in 2024.