From Lisbon to Berlin, center-left parties are breaking the taboo on pacts with the far left.

The formation of an all-left city government in Berlin that includes the once-communist Die Linke follows a pattern: center-left parties across Europe are increasingly willing to team up with their rivals on the far left.

Germany’s Social Democrats shunned Die Linke for decades. The two parties disagree on EU and industrial policy, NATO membership, relations with Russia and welfare.

The alliance in Berlin is only the second time in German history the two have shared power.

The Social Democrats earlier formed a coalition with Die Linke and the Greens in Thuringia, but that administration is led by Die Linke and therefore considered less of a national precedent.

Electoral necessity

It’s not that the Die Linke has come around to center-left politics. It’s an electoral necessity that the Social Democrats are reevaluating their position on left-wing pacts.

In Thuringia, the Social Democrats have for years placed third in state elections, behind the Christian Democrats and Die Linke.

The latter is more popular across the former East Germany, where one of the two parties that preceded it ruled under communism. Nationally, it gets between 8 and 10 percent support.

The Social Democrats, on the other hand, still get most of their votes from the industrial heartlands of the former West Germany, including Bremen, Hamburg and the Ruhr.

But their support nationwide has dropped from 40-percent highs to around 25 percent in the last two elections. Centrist voters have switched to Angela Merkel’s Christian Democrats; leftwingers have defected to Die Linke and the Greens.

The three left-wing parties could have formed a government in 2013. They won a combined majority of 320 seats in the Bundestag.

Instead, the Social Democrats went into coalition with Merkel’s Christian Democrats.

The polls don’t suggest voters will reward them for this decisions in 2017. Social Democratic support is in the low twenties.

Support for Die Linke and the Greens is stable: each party is polling around 10 percent.

That means the Social Democrats could face the same choice next year as they did three years ago: continue the grand coalition with Merkel or lead a left-wing government of their own.

Same dilemma

Social democrats across Europe face the same dilemma.

Spain’s this year had to decide whether to allow the right-wing People’s Party of Mariano Rajoy to stay in power or join hands with the anti-establishment Podemos movement, which won nearly as many seats in June’s election as the Socialist Workers’ Party.

The socialists debated for months. Like Die Linke in Germany, Podemos holds views on the EU, NATO and industrial policy that are way outside the mainstream and spook middle-income and small-town voters. But they are also seen as authentic and forthright, unlike the Socialists, who have wavered between supporting and opposing Rajoy’s austerity program.

In the end, the Workers’ Party couldn’t bring itself to form a government with Podemos and threw its support behind Rajoy. But there is a minority in the party that believes left-wing collaboration is inevitable.

Social democrats in Portugal already made that choice.

As in Spain, the center-right won a plurality, but not a majority, of the seats in parliament there. The Socialist Party under António Costa placed second. They could do one of two things: allow the right to remain in power or do a deal — for the first time in their history — with the Communists, Left Bloc and Greens.

Costa opted for the latter and it has gone surprisingly well. Politico described Portugal as an “oasis of stability” earlier this month. The far-left parties have been willing to compromise. Costa is praised by the electorate more broadly for presiding over an economic recovery. (Which owes something to the liberal policies of his predecessor, but that’s another discussion.)

Restoring the social democratic coalition

There is an ideological argument for left-wing unity as well.

Joop van den Berg, a Dutch political scientist, has argued that the traditional social democratic coalition, between workers and the intellectual middle class, is breaking down. The former are switching to reactionary populists on the left and the right; the latter are drawn to cosmopolitans in the center.

Left to themselves, social democrats tend to either technocracy or radical utopia, said Van den Berg.

The first has happened in Germany and the Netherlands: social democrats there provide competent, pragmatic leadership but fail to inspire voters.

If it weren’t for them, though, working-class voters could tend to authoritarianism.

We see this in France, where the unpopularity of the ruling Socialist Party has been a boon to Marine Le Pen’s Front national.

We’ve seen it in the United States, where disaffected blue-collar voters in states like Michigan and Ohio switched from the Democrats to Donald Trump in November.

What happens then, Van den Berg warned, is polarization, the disintegration of party formation and a weakening of collective solidarity.

That’s why it’s not only in the interest of social democrats to breathe new life into their coalitions; they need to succeed for the sake of liberal democracy itself.

Risks

A left-wing strategy is not risk-free. The recent experience of the British Labour Party shows that social democrats can become irrelevant if they lurch too far to the left.

But going into coalition with a far-left party is not the same thing as becoming one.

The strategy may not work everywhere. Die Linke and Portugal’s far-left parties have been around for years and governed locally. Podemos in Spain and the Five Star Movement in Italy, by contrast, are new and inexperienced. They are internally divided between pragmatists and hardliners. A coalition with them could be rocky. But so long as they’re on the outside, they keep their hands clean and appeal to voters who pine for ideological purity.