Brussels - Three months from the European Parliament election, the assembly's dominant centre-right bloc has been weakened by a rift over how to handle Hungary's populist leader Viktor Orban.
The conservative European People's Party produced the current president of the European Commission, Jean-Claude Juncker, and polls suggest it will remain parliament's largest group.
But the start of EPP flagbearer Manfred Weber's campaign has been overshadowed by Orban's increasingly strident attacks on Juncker and on the European institutions themselves.
The party, and Weber, now face mounting calls to expel the Hungarian nationalist from the EPP from those who argue his authoritarian tendencies run counter to European values.
And the battle within the EPP mirrors the broader confrontation across the continent between the mainstream federalist parties and, in several countries, a rising populist wave.
Weber, a German from the Bavarian conservative CSU party, has thus far defended the EPP's decision to allow Orban's Fidesz to swell its ranks in the Strasbourg assembly.
But the Hungarian government's direct attack on Juncker's Commission -- with the launch of an anti-immigrant, eurosceptic campaign -- has increased the pressure on him to act.
"You too have a right to know what Brussels is preparing," the government's campaign material declares, along with images of Juncker and liberal US billionaire George Soros.
"They want to bring in the mandatory settlement quota; weaken member states' rights to border defence; facilitate immigration with a migrant visa," the government claims.
This drew a furious response from the European Commission spokesman, who said the campaign "beggars belief" and branded it a "ludicrous conspiracy theory".
But it also stirred a political response from the ranks of Orban's fellow EPP leaders.
"The claims made in the campaign are deceitful, misleading and are not based on facts," tweeted Joseph Daul, the French president of the centre-right party.
"I strongly denounce Hungary's attacks and baseless conspiracies against President Juncker, who is a true Christian Democrat and a real European leader".
Weber retweeted Daul's broadside, but neither party leader called for Fidesz to be expelled.
Juncker's view was already known. He said last year that Orban's party has no place in the EPP.
- Weber's dilemma -
The EPP chose Weber as its leading candidate, with the support of Germany's Chancellor Angela Merkel, at a party conference in Helsinki last November.
The MEP's main rival, former Finnish leader Alexander Stubb, had argued for the party to distance itself from Orban, but delegates favoured Weber's call for party unity.
Now, however, he is under fire for his lack of charisma, and opinion polls predict the EPP will lose seats in May's vote, even if it remains the biggest bloc.
"He's trapped. If he opens the debate, he forces the entire party to take a position," argues Eric Maurice, from the Schuman Foundation think tank in Brussels.
To expel Fidesz from the group, at least seven EPP member national parties would have to formally write to request this. None have yet done so.
Nevertheless, Gunnar Hokmark, head of the Swedish contingent, has written personally to Weber to argue that Orban's case should be debated at a party meeting next week.
And Othmar Karas, head of the EU parliament candidate list for the Austrian conservative OVP, on Wednesday called again for Fidesz to be suspended from the group.
But the biggest national group, the Germans, are more reticent.
"If we push him out he'll fall into the arms of Salvini," argued a German MEP from Merkel's CDU, speaking on condition of anonymity.
Italian deputy prime minister Matteo Salvini, leader of the far-right League, has made no secret of his efforts to form a eurosceptic front during the campaign.
He has already been in touch with Orban and French right-winger Marine Le Pen seeking an alliance.
Without Fidesz, the EPP will lose 12 of what are currently its 217 seats in a parliament that will be 705-strong after Brexit.
Weber will need at least half of the MEPs to back him if he is to make good on his ambition to succeed Juncker as head of the Commission.
He could do this by forming a broad centre-ground alliance, but with both the EPP and the Socialists expected to lose ground, he will need liberal backing.
And if he's seen as Orban's ally, he won't get it. "His lack of reaction makes him even less legitimate in the eyes of the Greens, Social Democrats and Liberals," Maurice told AFP.
"All this increases the chances of an alternative candidate," he argued. Waiting in the wings is Michel Barnier, former French minister and now EU Brexit negotiator.
By Céline Le Prioux