In mid-May, the European Parliament's top civil servant Klaus Welle joked about the so-called Spitzenkandidaten - where lead candidates are selected for the next European Commission presidency.

"Our biggest success in the English language was 'Kindergarten'. Now we are doing 'Spitzenkandida't," quipped the German secretary-general to a round of laughter at the Norway House in Brussels.

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Fast forward to this week, post EU elections, and some of the heads of state and government meeting over a dinner in Brussels are hoping to consign that German word to the history books, as the battle for the top posts of the EU institutions kick off in earnest.

The stage is now set for a tussle of powers between the European Council and the European Parliament on who and how the next president of the European Commission will be selected.

On Tuesday (28) at around noon, the first warning shot was fired by the European Parliament, after political group leaders from the centre-right European People's Party (EPP), the centre-left Socialists & Democrats (S&D), the Greens, and the far-left GUE all joined forces to defend the Spitzenkandidat process.

"The majority of the groups is in favour of the spitzenkandidat position, which is the position of the European Parliament," announced European Parliament president, Antonio Tajani.

A statement endorsed by all four groups, following a morning meeting among them all, cited Treaty articles 17.6 and 17.7 in defence of the spitzenkandidat.

It also said a lead candidate is anyone who made their "programme and personality known prior to the elections, and engaged in a European-wide campaign."

That narrows the list down to the handful candidates already put forward by the individual groups, such as Frans Timmersmans for the socialists, Margrethe Vestager for the Liberals, or Ska Keller for the Greens, among others.

No 'parachuting-in'

But the ultimatum also means any surprise nominations by the European Council will be rejected by the European Parliament, causing a possible long-drawn out spat between the EU institutions.

Boosted by an over 50 percent voter turnout, the highest in 20 years, the European Parliament now sees itself as more self-assertive.

"Today is day of a basic decision on whether the parliament is a place for the decision-making process, that is today's date and the most important message," said Manfred Weber, the EPP chair who expects to become the next president of the European Commission.

However, notably absent from the declaration was liberal leader of the Alde group, Guy Verhofstadt.

The Belgian politician had instead issued a press release slamming the spitzenkandidat as undemocratic given the lack of transnational lists.

"A 'Spitzen' candidate that you cannot vote for in the whole of Europe is simply not serious," said Verhofstadt, in a statement.

Bullish Bullman

Udo Bullman, the leader of the socialist S&D, appeared sympathetic to Verhofstadt's concerns.

He told reporters that while his group also backs the concept of transnational lists, he doubts member states will select someone who was not a Spitzenkandidat.

"If I were the European Council, I would realise that this is a very self-confident European Parliament that has been elected," said the German socialist leader.

Meanwhile, the coalition building of a majority among the different groups in the European Parliament will likely factor into the European Commission presidency.

The point was driven home by Philippe Lamberts, co-president of the Greens.

He told reporters a stable majority in the parliament that straddles a political programme endorsed by the next European Commission is also in the interest of the Council.

"If the Council wants to achieve anything, it needs a stable majority in the European Parliament and conversely, if we want to achieve anything in the parliament, we need to have a majority in the Council to work with us," he said.