Spain's Senate on Wednesday followed the example of the lower house of Parliament and approved a constitutional amendment aimed at ensuring budgetary stability.

Members of both chambers will now have until Sept. 22 to request a referendum on the proposed change to Article 135 of the 1978 Spanish Constitution.

A plebiscite would only be held if at least 26 senators or 35 members of the lower house asked for it.

The proposal, first introduced Aug. 22, is the fruit of a pact between the governing Socialists and the conservative main opposition Popular Party, currently leading in the polls for the Nov. 20 general elections.

Socialist and PP senators secured the bill's passage through the upper house after defeating motions from leftist and regional parties demanding a referendum.

The amendment would be the first change to Spain's post-Franco democratic charter since 1992, when the constitution underwent minor adaptations to accommodate the European Union's Maastricht Treaty.

Tuesday saw nationwide protests against the amendment organized by Spain's two largest labor federations, leftist groups and elements of the May 15th Movement, which was behind massive mobilizations earlier this year to denounce austerity policies and demand "real democracy."

Critics of the budget amendment say Spanish voters should be allowed to have their say on an issue of such importance.

The speed of the process and suspicions that the measure is being dictated to Madrid by the European Central Bank and by Germany have spurred criticism even in the heart of the Socialist party.

Prime Minister Jose Luis Rodriguez Zapatero, who announced months ago that he would not seek a third four-year term, insists the amendment will send a positive message to financial markets worried about Spain's large public debt.

The text of the constitutional amendment calls for a deficit approaching zero by 2020, but leaves the details for a subsequent piece of legislation to be debated and passed by the Parliament elected in November.