Speaking on Saturday as leader of his Socialist Party, at two-day political event organised by its German sister organisation, the Social Democrat Party (SPD), Costa stressed the need to mobilise the broad left.

"Now that we have already left the excessive deficit procedure [of the European Union], now that Greece has come out too, now is the time to complete the economic and monetary union,” Costa said, before going on to refer to Italy’s populist deputy prime ministers. “It is on the sunny days that the roof is fixed and we must fix it before [Matteo] Salvini and [Luigi] Di Maio bring us new days of rain."

Costa cited Portugal as an example that "the best way to combat populism is to ensure that there is always a democratic alternative" – a reference to the way his minority Socialist government has survived thanks to agreements with other parties.

"Of course, democracy by its essence means compromise and that the current electoral fragmentation in European liberal democracies requires compromise solutions for government,” he went on. “You here in Germany have the Grand Coalition, in Greece Alexis Tsipras has the support of the Independent Greeks, and we in Portugal have a parliamentary agreement with parties to our left – the Left Block, the Communist Party and the Ecologist Party ‘The Greens’.”

The leader of the PS also pointed to the "key to the success and stability" of the political solution in Portugal: "Each party maintained its own identity without the temptation to occupy the political space of others, which makes us risen not at the expense of our Partners, but of our opponents. "

He stressed that "more than a single currency, more than a single market, Europe is a community of values" and went on to illustrate this in a way that brought laughter from his audience.

"Sharing a common house requires sharing common rules as it does in any family,” he said. “We can all sit at the table to share a meal, but we don't all have to eat the same. I, prefer a good Eisbein [ham hock] with sauerkraut, but my son, who is on a diet, eats boiled fish and my daughter-in-law a vegetarian dish."

After Costa, it was the turn of Tsipras to speak, also highlighting the importance of seeking democratic solutions, to avoiding extremism.

"It is our responsibility to avoid this nightmare, it is very dangerous, not for parties, but for people,” he said. “The left needs to speak the language of the people [and] work together."

The two visiting speakers were introduced by the SPD’s leader, Andrea Nahles.

The SPD has steadily lost popularity since it has been in coalition with the Christian Democatic Union (CDU) of Germany’s chancellor, Angela Merkel. In the last regional elections, in Bavaria and Hesse, it slipped to around 10% of the vote.

Meanwhile, the far-right Alternative for Germany (AfD) has been gaining ground, and now has seats in all 16 regional parliaments in Germany.