Manfred Weber of the centre-right European People's Party set an ultimatum Tuesday for Hungary's populist Prime Minister Viktor Orban and his Fidesz party to avert exclusion from the group.
After months of rising tensions, Weber, the EPP's leading candidate to take over as head of the European Commission later in the year, told Germany's daily Bild he was setting three conditions to remain on board.
"Viktor Orban must stop the anti-Brussels campaigns of his government once and for all," the German politician said.
Secondly, he must "apologise to the other member parties of the EPP party family".
And finally, "it is important that the CEU university founded by (liberal US-Hungarian billionaire) George Soros remains in Budapest, that its existence be ensured there and that it is able to issue US diplomas".
In December the CEU announced it was moving most of its programmes out of Budapest to Vienna after it said it was targeted by government legislation steamrolled through parliament in 2017.
Weber said Orban has, until the end of March, "the chance to take action on these three points and to show that he values the EPP", calling them "decisive issues regarding democratic principles and political style".
On Monday, three months ahead of the European Parliament elections, EPP members officially requested the exclusion of Hungary's nationalist leader and his Fidesz party from the group.
The matter is to be discussed at an EPP meeting on March 20.
The move came following a series of measures that raised doubts within the assembly's dominant bloc about Orban's commitment to EU values.
Adding to the strain was a campaign in Hungary targeting European Commission President Jean-Claude Juncker, also of the EPP, accusing him of backing illegal migration.
Budapest has since said it would stop agitating against Juncker.
The EPP is the biggest party in the European Parliament and comprises the main centre-right movements in Europe, including German Chancellor Angela Merkel's CDU and France's Les Republicains, which have not moved against Orban.
Looking to the March 20 gathering, Weber said he would make "a final effort to keep Viktor Orban and Fidesz in the EPP" but stressed that the group's values were "not negotiable".
"The ball is in Budapest's court. We need clarity this month," he said.