Spain’s Catalonia will hold elections for its regional parliament on September 27, its president, Artur Mas, announced on Wednesday. The main parties want to use the vote as a proxy for a referendum on independence opposed by Spain’s central government in Madrid.

The Catalan election would take place just two months before a general election. Prime Minister Mariano Rajoy, who has vowed “never” to negotiate the unity of Spain, will be fighting off a challenge from far leftists who are ahead of his ruling People’s Party in the polls.

Mas’ right-wing Convergence and Union alliance is expected to win the election in Catalonia, followed by the Republican Left.

Mas had tried to persuade the Republicans to come up with a single electoral list but the leftists declined. “It would be a betrayal of our supporters if we leave the banner of social justice and the fight against corruption in the hands of those who want independence,” party leader Oriol Junqueras said last month.

Junqueras also said he was in favor of continuing his coalition with Mas, however.

“We will run with different lists but with a common national road map,” the regional president said on Wednesday.

If the two parties win a majority between them, they promise to interpret the outcome as a de facto vote for independence. Mas has said that the Catalan government would be able to immediately start building up “state-like structures” and prepare to secede within a year and a half after the vote.

In November, more than 80 percent of Catalans voted for statehood in what was dubbed a “citizens’ consultation” after Spain’s highest court had struck down a planned referendum as unconstitutional. Roughly half of the region’s 5.4 million eligible voters participated.

The central government’s refusal to negotiate increased autonomy for what is Spain’s richest region has fed separatist sentiment. A poll lost month found only a slim majority would vote to stay part of Spain but support for independence is up from 13 percent in 2005.

Catalonia is richer than the rest of Spain and a net contributor to the country’s public finances. It has 16 percent of the Spanish population but produces more than a fifth of national economic output, giving it an economy the size of Denmark’s. An estimated $21 billion in Catalan taxes, equivalent to 8 percent of the region’s gross domestic product, is invested in other regions.