Spain’s Socialists and far-left party Unidas Podemos have reached a coalition deal, La Vanguardia first reported on Tuesday.

“An agreement has been reached for a coalition government between Unidas Podemos and the Socialists,” UP lawmaker Alberto Garzon said in a tweet.

The agreement is a political milestone.

The failure of the two parties to reach an agreement in April led to new elections in November, in which both parties sustained electoral losses. Since the country’s transition to democracy in 1976, Spain has not had a coalition government.

But this alliance was only the first step and perhaps not the hardest.

The combined parliamentary power of the two parties will not suffice. The Socialists gained 120 seats and Unidos Podemos 35. A majority in the 350-seat parliament requires 176 seats. The coalition is 21 seats short.

The small leftist Mas Pais may add its three seats to the tally. To this one may add the six MPs of the Basque Nationalist Party (PNV). Smaller parties of the regional bloc are likely to join. The big question is whether the left will secure centre-right or Catalan support.

The alliance would have a functioning parliamentary majority if it were joined by 15 MPs pf the Catalan leftwing separatists Esquerra Republicana de Catalunya (ERC). But their support will accentuate the divisions within the Spanish left over the question of autonomy  and the right to self-determination of Spain’s constituent nationalities.

Thus far, the Socialists have proved uncompromisingly unitarian. ECR could demand a Scottish type independence referendum, sanctioned by the state, as a price to pay for their support. That is why the second step towards a coalition government will be the harder one to make. This would bolster the conservative backlash.

The alternative for the Socialists is to turn to the centre-right for a German-style grand coalition. For the moment that appears to be a political dead-end. The 10 seats of the Centre-right party Ciudadanos may not be forthcoming.

Following the resignation of their leader, Albert Rivera on Tuesday, the party confirmed they would be unwilling to join a left-leaning government, calling on the Socialists to join a grand coalition with the Conservative People’s Party (PP).

“We appeal to the responsibility of PSOE and PP to reach a moderate and constitutionalist agreement with Ciudadanos based on state deals that are good for our country,” the party said.