President Zoran Milanovic’s announcement on Thursday that he would tell Croatia’s ambassador to Nato to vote against the Nordic expansion, makes him the second alliance leader to raise concerns after Turkey’s Recep Tayyip Erdogan.
Mr Milanovic’s objection is not to Sweden and Finland, but to Croatian Prime Minister Andrej Plenkovic, a political rival, who supports Nato expansion and has clashed with the president on numerous domestic issues.
Mr Plenkovic, in turn, has accused the head of state of taking a pro-Kremlin stance by opposing the expansion of Nato, a position that drives much of Russia's foreign policy.
The feud is causing uncertainty over whose orders the ambassador will ultimately accept if the dispute is not settled. Croatia’s constitution says the president should co-operate with ministers in making foreign policy.
Nato membership is granted by consensus rather than formal voting, meaning a lack of consent from Turkey and Croatia could effectively hold up the process for Sweden and Finland until their objections are answered.
Nato Secretary General Jens Stoltenberg, and leading member states such as the US, Britain and Germany, have given enthusiastic support to the two Nordic applicants, who already co-operate with Nato and take part in drills.
Both countries made a historic break with decades of military neutrality to announce they were seeking Nato membership in what they described as a changed security environment created by Russia’s invasion of Ukraine.
But Mr Milanovic, the sceptical president, said the war in Ukraine was “not a burning issue” for him compared with his greater political priority of securing the rights of ethnic Croats in neighbouring Bosnia and Herzegovina.
He said he would write to Mr Stoltenberg explaining why he was withholding support for Nato expansion until he gets his way with Mr Plenkovic on the Bosnian question.
“If we ratify this without firing a single shot, we will be selling our family silver, the only instrument and leverage we have at this moment to solve a serious problem,” Mr Milanovic said.
“That is not an act against Finland and Sweden, but it is for Croatia.”
The prime minister fired back by saying Mr Milanovic appeared to some as a “useful fool” acting in Moscow’s interests and had spoken out against Nato expansion in general since before he made the Bosnian question his focus.
"Unlike him, we have a policy that is responsible, it takes into account the interests and reputation of Croatia, as a country that has painstakingly built its international rating,” Mr Plenkovic said.
“We need to protect the interests of Croats in Bosnia and Herzegovina, but in a wiser way, not in this way.”
Hopes of a quick approval process have already been dented by Turkey’s objections, which centre around the alleged activities of Kurdish militants in Sweden and Finland.
Swedish Foreign Minister Ann Linde said on Friday that there was "vastly spread disinformation" on that question, saying Sweden in the 1980s had been one of the first countries to list the militant PKK as terrorists.
Mr Milanovic says Croatia should “follow Turkey’s example” by setting a high price for its support.
The president is demanding that Bosnia amend its electoral law to protect minority Croats. Mr Plenkovic wants this too but says he is seeking a more consensual solution brokered by the European Union.