In an interview published jointly by six major European newspapers on Monday, the Prime Minister of Spain stated emphatically that "I do not want, but am also unable to... authorize a referendum [in Catalonia] for the simple reason that national sovereignity corresponds to the Spanish people". This statement is, due to its very incorrectness, unfortunate. Constitutionally speaking, Mr. Rajoy does, indeed, have the right to not want to authorize a referendum in Catalonia. However, leaving aside the question of his personal wishes, the Constitution does, in fact, give him the power to call a referendum directly or allow the Generalitat de Catalunya government to call one.
Spanish law allows for five ways to undertake a referendum. In first place, Article 92 of the Constitution provides for the calling of referendums on political decisions "of special importance", when proposed by the Prime Minister and following the authorization of the Spanish parliament. Since this article does not regulate the territorial reach or the institutions that can take part, we must conclude that it allows for referendums at the Autonomous Community level and the participation of governments such as the Generalitat.
In second place, Article 150.2 of the Constitution provides for the possibility to delegate or transfer the power to call referendums to the Autonomous Communities. This delegation of powers only requires approval by an absolute majority (which is currently held by the Popular Party) in the Congress of Deputies. This possibility is, in fact, the most similar scenario to the case of Scotland.
In third place, the Generalitat can call a referendum, with the prior approval of the central government, under the Catalan law 4/2010 of popular consultations through referendums, which was passed in order to "encourage participation and increase the quality of democracy by promoting the implementation of mechanisms of citizen participation". It is true that the law authorizes the Government to act only in areas within its competence. But it is also true that the Constitution allows Autonomous Communities to ask the central government to adopt a draft law to reform the Constitution (Article 87) or to send a specific bill to Congress with the same purpose (Art. 166). Therefore, a referendum such as the one under debate now provides a perfect fit with the logic that enables the Generalitat to start a process of revision of the constitution.
In fourth place, the Catalan Parliament is currently debating a law regulating popular consultations other than referendums (following the definitions laid down by the Spanish Constitutional Court) that will allow it to undertake this type of consultation without the need for prior approval by the Spanish government. The Spanish government could, of course, ask the Constitutional Court to suspend the law once it is passed and to declare it unconstitutional. However, this action would, once again, be a matter of political will rather than a question of legality.
Finally, the catalan government could call for a reform of the Constitution to call a referendum. The Spanish government could not refuse to consider it for "constitutional" reasons but rather for purely political considerations, since the 1978 Constitution contains no provision that would prevent its partial or total reform.
In more general terms, the Spanish government often cites Article 2 of the Constitution, which speaks of the "undissolvable unity of the Spanish State", in order to argue that the type of consultation that the Generalitat wishes to hold is unconstitutional. Notwithstanding the fact that the Article 2 does not, at any time, define the territory or population that constitute the Spanish State, the position of the Spanish Government is based on an erroneous assumption. As its name suggests, the consultation is an instrument to ask Catalan on the political status of Catalonia and is not necessarily a tool to change the Constitution automatically.
A comparison with the case of Quebec is pertinent here. As the Canadian Supreme Court ruled in 1998, although the Canadian constitution (like the Spanish one) regulates the right to self-determination, the application of a general democratic principle made a consultation with the citizens of Quebec a perfectly legitimate act under the condition that, in the case of a victory for the yes vote, Canada and Quebec negotiate a constitutional reform to enable the secession. This way of conceiving law and democracy seems perfectly applicable to Spain. All that is needed is political will.