A fierce fight is brewing among EU leaders over the next European Commission president — not who, but how.

The so-called lead candidate — or Spitzenkandidat — process, used for the first time in the appointment of Jean-Claude Juncker in 2014, is not written in the EU treaties. Rather, it is little more than a handshake agreement among the main European political parties, the European Council and the European Parliament, over how to interpret a vague treaty provision.

The fight over whether to use it during the next European election in 2019 highlights a seemingly absurd situation: The EU, paragon of democracy, has no agreed-upon process for choosing its most powerful official. Resolving how to do so goes to the heart of the credibility of the European project.

The EU treaties require the Council, acting by qualified majority, to nominate for Parliament’s approval a Commission president “taking account of the results of the European Parliament election.” How to fairly take account of the results is at the core of the dispute.

The Spitzenkandidat process awards the Commission presidency to the party winning the most seats in Parliament. Critics say it’s a handshake deal, minus the handshake — an opaque process that strips the Council and Parliament of their authority and duty to pick the best candidate, and forces them instead to accept the winner of a process driven by party insiders.

“Who should decide about the chief of the Commission? All member states, not somebody saying ‘ah, this is our Spitzenkandidat" — Czech Prime Minister Andrej Babiš

A majority of the European Council, including French President Emmanuel Macron as well as the leaders of the Czech Republic, Hungary, Lithuania, the Netherlands, Poland, Portugal and Slovakia have voiced opposition to the process.

But supporters — including Irish Prime Minister Leo Varadkar, as well as Juncker and his powerful chief of staff, Martin Selmayr — say the EU’s credibility depends on maintaining the process, in which the political parties vying for representation in the European Parliament put forward nominees to campaign across Europe and challenge each other in debates. The winner is the nominee whose party wins the most seats in the European Parliament.

The largest parties, including Juncker’s center-right European People’s Party (EPP), the center-left Party of European Socialists and Democrats (PES), and the Alliance of Liberals and Democrats for Europe (ALDE), have all committed to follow the process next year. The European Parliament is expected to adopt a resolution declaring it will only approve a Spitzenkandidat — meaning someone who has the formal backing of a party grouping — as the next Commission president, though there is no legal way to control how MEPs vote.

But nothing requires the European Council to follow the Spitzenkandidat procedure and, in fact, some analysts say committing in advance to the process would be illegal — a violation of provision of Article 17(7) of the Treaty on the European Union, which specifies that the Council, acting by qualified majority, must nominate a candidate, to be confirmed by a majority of the Parliament.

“Let’s make permanent the Spitzenkandidat system, and democratize choosing candidates for other leading positions within the EU" — Irish Prime Minister Leo Varadkar

Supporters point to a Commission recommendation released in March 2013 on “enhancing the democratic and efficient conduct of the elections to the European Parliament” that backs the process.

The fight will be one of the main agenda items at a summit of EU leaders next month. Battles lines are already being drawn, with some leaders on the European Council saying they feel no obligation to repeat a process tried only once before that seems to favor Germany and France, the two biggest EU countries.

“We cannot be in the position that we have nothing to say, that there are only two big nations and the Commission, which are really deciding about everything,” Czech Prime Minister Andrej Babiš told POLITICO. “Who should decide about the chief of the Commission? All member states,” he said. “Not somebody saying ‘ah, this is our Spitzenkandidat.’”

Macron has made an impassioned case for “truly European elections” and pushed hard for a new process by which MEPs are elected on transnational lists. But he does not back the Spitzenkandidat process, which virtually guarantees a victory for a big, traditional party, according to officials close to him.

European Council President Donald Tusk, who is a former Polish prime minister from the EPP, expressed his own reservations about the Spitzenkandidat process in a speech to the Parliament last fall. While many MEPs like the concept because it appears to give them more sway over the outcome than the Council, Tusk warned that wasn’t necessarily the case.

“As for the Spitzenkandidat, it was not the European Council that was the weak link in 2014,” Tusk said. “Jean-Claude Juncker was elected with 26 votes against 2 votes in the European Council, while in the Parliament he got 422 votes out of 729 votes. So it is easy to imagine the situation that it will be more difficult for the winning Spitzenkandidat to win sufficient support in the new European Parliament than in the European Council.”

While the Spitzenkandidat process, as the name suggests, is a German concept, it is not universally supported in Germany. Its main champion in 2014 was Martin Schulz, then president of the European Parliament and a leader of the Social Democrats. Chancellor Angela Merkel, of the rival EPP, was not an advocate of the process, though she ultimately backed Juncker over Schulz in 2014.

Fans of the process express zealous enthusiasm for a system that appears to add an element of participatory democracy to choosing the president of the Commission, long derided as a bastion of unelected bureaucrats.

Supporters of the Spitzenkandidat process, who are generally proponents of greater European political integration, have sought to prove that it has legal or even constitutional basis, with an array of fact sheets, reports and even doctoral theses — though none has settled the debate.

Selmayr, who managed Juncker’s successful campaign in 2014, described the process as a core of EU democracy. “One should never formalize constitutional reality (“Verfassungswirklichkeit”), but live it,” he told POLITICO.

In a recent speech to the European Parliament, Irish Prime Minister Varadkar said: “Let’s make permanent the Spitzenkandidat system, and democratize choosing candidates for other leading positions within the EU.”

Ireland, Italy, and Spain appear to be among a small group of EU countries willing to endorse the process in 2019.

Manfred Weber, the president of the European People’s Party, the largest group in the Parliament told POLITICO that appointing the next Commission president via the Spitzenkandidat process was “a matter of transparency and democracy.”

“We want European voters to have their say in the choice of Commission president,” he said. “They don’t want this decision to be taken behind closed doors anymore.” Weber said the Parliament would battle the Council to retain the process. “We are ready to fight,” he said. “We will not accept any candidate that is not a Spitzenkandidat.”

It is far from clear that voters care. According to official statistics, turnout in European elections has fallen consistently since 1979, and did not increase with the introduction of the Spitzenkandidat process in 2014.

Some critics note that an anti-EU party could win a plurality of seats, while a coalition of pro-EU parties still controlled an overall majority. In that case, they ask, should the Council and Parliament be forced to choose an anti-EU candidate?

Critics also note that the Spitzenkandidat effectively eliminates from consideration any sitting head of state or government because it is unrealistic to expect someone to resign from national leadership to campaign for the Commission post.

In Brussels, Council officials, fearing a divisive fight, are casting about for a compromise, having concluded that EU leaders at most can make a political — not legal — commitment to the Spitzenkandidat process.

Babiš said he had never heard of the process until becoming Czech prime minister in December. “I never heard this name before,” he said, laughing. “Now I know what is a Spitzenkandidat.”