Members of the European parliament will challenge Hungary’s prime minister, Viktor Orbán to his face on Tuesday in a debate about the rule of law in the country.

MEPs will decide on Wednesday whether to trigger a little-tested EU rule of law procedure, which would mark a significant escalation in the bloc’s response to Orbán. Since the prime minister returned to power in 2010 he has presided over a weakening of press freedom, an erosion of judicial independence and the alleged misspending of EU funds by his friends and family.

But it is unclear which way the vote will go. Orbán is likely to benefit from some support from his political group, the European People’s party (EPP) – the pan-European alliance of centre-right politicians that includes Angela Merkel and Jean-Claude Juncker. Many other MEPs, including the UK’s Conservatives, see the action as a politically motivated encroachment on a state’s rights, and are likely to oppose it.

Orbán, who was re-elected in April, is expected to give a pugnacious performance when he addresses MEPs in Strasbourg on Tuesday. He has long accused the EU of overreaching its powers – a central charge of his “Stop Brussels” campaign.

Two-thirds of MEPs are required to trigger the EU’s rule of law procedure, known as article 7. The Dutch Green MP Judith Sargentini, who has been leading the process through the European parliament, said the outcome was too close to call.

Urging colleagues to support the move, she said: “The facts don’t lie. I cannot conclude anything else than that there is a risk of a serious and systematic breach of European values that we all share.

“This makes it so important that all members of this parliament must speak out,” she said. “Abstention is the wrong thing to do because it is seen as a no vote.”

The EU rule of law procedure is intended to stop states backsliding on democracy and fundamental rights, but is fraught with political dangers.

Poland is the only EU country subject to this process, after the commission found that changes to its judiciary constituted a fundamental threat to the rule of law.

Brussels is reluctant to move quickly on the Polish case, fearing any misstep could hand a propaganda victory to Warsaw. Hungary has already said it will veto any move to strip Poland of voting rights, the ultimate sanction.

Sargentini acknowledged the political difficulties but said countries could not ignore that Orbán was moving “full-speed” away from democracy.

But Ágoston Mráz, director of the government-funded Nézőpont thinktank, said the process would only boost Orbán’s narrative at home. “The procedure is a gift to the campaign: an external attack on Hungary and its independence,” he said.

The Hungarian issue is causing friction in the EPP grouping. Uneasy with the leadership’s support for Orbán, smaller delegations, such as the Dutch and Luxembourgish, are considering quitting the group.

The EPP’s leader, Manfred Weber, who launched a bid to become European commission president this week, said he was “absolutely concerned” about Hungary’s NGO law and the fate of the Central European University.

Hungary has been referred to the EU’s highest court over the two issues: a higher education law that could force the closure of the Central European University and an edict requiring NGOs to label themselves as “supported” from abroad.

Weber defended his decision not to expel Orbán’s Fidesz party from the EPP group by drawing a comparison with when David Cameron withdrew the UK’s Conservative party voluntarily.

He said: “The starting point of Brexit was the decision of David Cameron to leave the EPP group … I don’t want to see the same happening with the east of Europe.”

The Conservatives, who have already been criticised for their refusal to support sanctions on Hungary, are expected to vote against the measures on Wednesday. Senior Tories say their stance is not tantamount to approval for Orbán’s policies, but they believe the European parliament should have no role in a member state’s political process.

Sargentini said: “Theresa May feels supported by Viktor Orbán in Brexit talks and that is very interesting.”

Additional reporting by Shaun Walker in Budapest