BRUSSELS/BUDAPEST (Reuters) - Hungarian Prime Minister Viktor Orban will attend a meeting of conservative officials from across Europe that may decide whether his party will stay in the main EU center-right political group where he has been accused of authoritarianism.

Slideshow ( 2 images )

Wednesday’s meeting of delegates from the European People’s Party could be the denouement of a years-long dispute between the populist, anti-immigration Orban and more mainstream, pro-EU parties in the EPP that accuse him of flouting the rule of law.

Thirteen member parties called for a vote on the Fidesz party’s continuing membership after it distributed posters depicting European Commission President Jean-Claude Juncker, an EPP member, as a puppet manipulated by billionaire George Soros into backing uncontrolled immigration into Hungary.

The stakes are high for both sides. Losing Fidesz’s legislators - currently there are 12 - could cost the center-right group its position as largest party in the European Parliament after May’s elections. Worse, other parties might follow.

But for Orban, being in a group containing German Chancellor Angela Merkel’s Christian Democrats (CDU) and venerable government parties from the Netherlands, Belgium and Scandinavia gives him access to the continent’s power brokers and confers a mainstream respectability that other populists lack.

The CDU has gone to great lengths to preserve relations with Fidesz, even as rights groups accused him of stoking ethnic hatred with anti-migration campaigns, and interfering with judicial independence.

But the posters, and Orban’s campaign against the private Central European University in Budapest that Soros founded, could have pushed things too far.

There are signs that the calculus is shifting for Orban as well: Hungary’s pro-government press have called for Fidesz to quit the EPP rather than endure “humiliating” negotiations.

“All the signals that are coming from Budapest suggest they are targeting a break,” said Andreas Nick, the CDU’s point-man on relations with Hungary in Germany’s parliament. “It looks as if they are really begging to be kicked out.”

Nick has described a meeting with a Fidesz official who asked him whether he “also got money from George Soros” after he had had expressed support the Central European University . “I showed him the door,” he said.

Orban has talked of shifting the EPP to the right. If that fails, he has suggested Fidesz could form an alliance with Poland’s Law and Justice (PiS).

It is also possible that the 260 delegates could hedge their decision, for example by suspending, but not expelling, Fidesz.

The challenge is most serious for Manfred Weber, a German ally of Merkel’s who is the conservative bloc’s lead candidate in the European Parliament elections and a possible successor to Juncker as European Commission chief - an ambition that could depend on whether he can keep Fidesz on side.

But unsuccessful attempts at mediation could undermine his authority and are a gift to other parties that accuse the EPP of being soft on what they call fundamental European values such as democracy and the rule of law.

“Viktor Orban has undermined freedom of the press in Hungary, forced a university to close and harassed NGOs,” said Ska Keller, the Greens leader in the European Parliament.

“Manfred Weber cannot be trusted as a candidate for the EU’s top job if he continues to defend Orban.”