The two largest parties in Spain’s Catalonia unveiled a road map for independence on Monday that would see the region secede no later than March 2017 if they win local elections due later this year.

The agreement was signed between regional president Artur Mas’ liberal Convergència Democràtica de Catalunya and the left-wing Esquerra Republicana. Mas’ conservative coalition partner, the smaller Unió Democràtica de Catalunya, did not join the accord, suggesting a possible split in the right-wing nationalist camp.

The plan would effectively turn the September election into a referendum on Catalan independence. If the two parties win a majority, they promise to draft a new constitution that voters would need to approve the following year. The regional legislature could then declare independence unilaterally.

The two parties are hovering around 20 percent in the polls each, meaning they would fall short of the 68 seats needed for a majority in the region’s parliament.

Spanish prime minister Mariano Rajoy, who faces national elections later this year as well, has vowed “never” to negotiate the unity of Spain. He said on Tuesday the road map was “bad for Catalans and the rest of Spain.”

An earlier independence referendum was ruled unconstitutional by Spain’s highest court.

In November, more than 80 percent of Catalans nevertheless voted for statehood in what was dubbed a “citizens’ consultation.” 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. Polls since the start of the year have shown that a slim majority of Catalans is more likely to vote to remain part of Spain but support for breaking away is up from 13 percent in 2005.