Efforts to restart the motor of European integration, already under threat from an expected populist surge at this week's EU elections, face another challenge from infighting over Brussels' top jobs.

Eurosceptics and nationalists are expected to gain seats in the EU parliament when results are announced on Sunday, but mainstream conservative, social democratic and liberal parties are still set to outmatch them.

The new majority, however, will not be able to return to business as usual until the union's national leaders agree on a new head of the European Commission, to replace Jean-Claude Juncker in November.

And here, their fragile unity will come under immediate pressure, senior Brussels insiders and expert observers agree.

"The sovereignists will remain marginalised at a European level by the parliamentary victory of pro-European forces," said former Italian premier Enrico Letta, chairman of the Jacques Delors Foundation.

"But we must be realists. There's a risk that this result will fall apart immediately if the debate about nominations begins with vetoes, obstruction and reprisals," he told AFP.

The first clash is expected to erupt on Tuesday, when the 28 EU national leaders meet in Brussels to discuss who should take the top jobs in Europe, including the prized role at the head of the commission.

Chancellor Angela Merkel has said she will back Manfred Weber, a fellow German and the lead candidate of the centre-right European People's Party, expected to remain parliament's biggest bloc.

French President Emmanuel Macron opposes both Weber and the whole idea of allowing the parliamentary vote -- the so called "spitzenkandidat" system -- to determine who is nominated to the role.

Many other national leaders share Macron's scepticism, but Weber plans to convince the mainstream parliamentary forces to insist that they will only ever ratify one of their own candidates.

The standoff could paralyse the process, and EPP supporters are pointing to the threat of far-right eurosceptics as they urge the leaders to fall in behind their spitzenkandidat.

"Matteo Salvini will have the biggest party in Italy, Le Pen will be number one in France and Nigel Farage's Brexit Party will be in the lead in the United Kingdom," a party official said.

The EPP is expected to have around 30 fewer seats in the new assembly, where a loose alliance of populists, eurosceptics and nationalists could add two dozen seats and swell its ranks to around 180.

Battle brewing

But, while the EPP is expected to form a majority coalition in the 751-seat chamber with the centre-left S&D and liberal ALDE, the populists will struggle to form a cohesive voting bloc.

Poland's Law and Justice (PIS) forms part, along with what remains of the British Conservative contingent, of the European Conservatives and Reformist (ECR) group, which opposes closer EU integration.

But they have not yet joined the frankly eurosceptic European Alliance of Peoples and Nations, backed by Le Pen and Salvini's League, nor Farage's populist Europe of Freedom and Direct Democracy.

Nevertheless, if centrists want to push on with the European project, they must beware an immediate rift between Macron and Merkel in the European Council, which represents the EU leaders.

"A battle is brewing between the European parliament and the Council over job nominations," said analyst Gianni Bonvicini in a report for his Italian thinktank, Affari Internationali.

Macron could very well gather enough support among fellow heads of government to block Weber at a June 21 summit, but then parliament could refuse to ratify the Council's alternative candidate.

The parties want to avoid a showdown, but they do not have long between Sunday's vote results and Tuesday's dinner summit to get a deal.

"Weber has 48 hours to save his nomination. He'll meet the leaders of the other political groups on Tuesday to try to impose the spitzenkandidat rule before the summit," an EU source told AFP.

Playing the system

EPP big-wigs will meet to confirm their support for the dour Bavarian, and the other parties will be asked to confirm they "won't vote for anyone from outside the process."

"If we give up on this principle, the European parliament is dead," one centre-right leader said.

This would effectively prevent the leaders from naming a candidate like EU Brexit negotiator Michel Barnier or competition commissioner Margrethe Vestager, who were not formal lead candidates.

The parliamentarians could also seek to undercut Macron further by backing liberal group leader and former Belgian PM Guy Verhofstadt as their new speaker - peeling him away from Macron's camp.

The EU treaty gives the job of nominating a candidate to the leaders, but Brussels insiders have grown adept at playing the system, and the heads of government failed to block the last winning spitzenkandidat.

"Merkel tried that in 2014, it didn't work out for her," Juncker said.

But is Macron succeeds in 2019 where Merkel failed before, and blocks the parliament's candidate, then EU politics is headed, in the words of the centre-right figure for"a summer of crisis".

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