The vice-president of the European Commission visited Budapest on Saturday with the message that supporters of Hungary's far-right government are being misled into xenophobia.
Frans Timmermans is the favourite to lead the European Socialists' party after the EU elections in May.
"I think the electorate of Fidesz are proud Hungarians, who have been misled by the leadership into believing that there are enemies everywhere," he said referring to prime minister Viktor Orban's ruling party.
"I believe the most important thing we can do for Hungarian society is to get rid of this phobia for others. It is not making Hungary strong, it is making Hungary weak."
Hungary's Socialist party also held their congress in Budapest on Sunday.
"Our message that we will deliver is 'stop Orban'," one of its MEPs told Euronews.
"We are fed up with the building of his system. If we can motivate opposition voters, then we can have a good result," István Ujhelyi said.
Orban has dismissed migration as an answer to his country's low birth rate. Instead he's offering Hungarians tax cuts and loans to have more children.
Most of the Balkan states are rapidly de-populating according to the UN’s Population Division.
“Several countries are expected to see their populations decline by more than 15 per cent by 2050, including Bulgaria, Croatia, Latvia, Lithuania, Poland, Republic of Moldova, Romania, Serbia, Ukraine and the United States Virgin Islands,” it stated in a report published in 2017.