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BRUSSELS (Reuters) - The European Union’s deputy chief executive Frans Timmermans will lead the center-left campaign in May’s EU parliamentary election, the party said on Monday, after fellow commissioner Maros Sefcovic stepped aside.

Sefcovic, who is vice president for energy on the European Commission, said he endorsed Timmermans, the executive’s first vice president, in his bid to succeed Commission President Jean-Claude Juncker next year.

Sefcovic, from Slovakia, said he stood down in the interests of party unity, avoiding a need for the Party of European Socialists to vote on who will lead its campaign.

Timmermans, a former Dutch foreign minister, promises to bring the EU closer to ordinary voters at a time when Britain’s pending exit is inspiring other nationalist movements to challenge European integration.

Monday’s announcement came three days before the center-right European People’s Party votes at a Helsinki convention to choose its lead candidate for May’s vote. They are widely expected to back Manfred Weber, the party’s German leader in the European Parliament, over ex-Finnish premier Alexander Stubb.

The parliament says that Juncker’s successor must be the lead candidate, or Spitzenkandidat in German, from a winning party. But many national leaders, who will nominate the next head of the Commission after May’s vote, say they reserve the right to put forward anyone they like for parliament’s approval.

That leaves it unclear how the battle for the EU’s top job will play out. Conservative French Brexit negotiator Michel Barnier or centrist allies of French President Emmanuel Macron, such as Danish EU commissioner Margrethe Vestager, are also cited as contenders. There is even speculation on German Chancellor Angela Merkel as a dark horse, notably since she announced last week that she will not seek re-election.

National polls suggest that the EPP will remain the biggest party in the EU legislature, followed by the center-left. But their informal “grand coalition” may well fall short of a majority, giving more influence to greens or centrists, as well as to growing anti-EU nationalist groups.