Catalans go to the polls at the end of this month to choose a new regional government in what is shaping up to be a showdown between the secessionists and central government in Madrid and between Catalans themselves, who are split on independence.

The majority of pro-independence groupings have come together as Junts pel Sí (United for a Yes Vote), a single-issue coalition that includes Artur Mas, the incumbent Catalan president. The poll has been billed as a plebiscite, and Mas has said he will declare unilateral independence if the group wins a majority of seats, even if it has not obtained a majority of the popular vote.

The first test will be the turnout on Friday, in what has become an annual show of force by the secessionists on Catalan National Day, as tens of thousands will converge on the capital to demand independence. This year the demonstration has been billed as “The Open Road to the Catalan Republic”.

In spite of polls showing waning support for independence, Raül Romeva i Rueda, the ex-communist who leads Junts pel Sí, says a unilateral declaration of independence is justified because “they [Spain] have beaten us with unjust laws and huge fines”. The Madrid government has refused to enter into a dialogue on independence and has adopted a hardline stance throughout. Its reaction to Junts pel Sí has been to rush through an amendment to the constitution so that any politician who declares independence can be imprisoned.

Mas says there is no option but to treat the election as a plebiscite, as central government has refused to allow a referendum. He says that if Junts pel Sí wins the Catalan elections on 27 September and goes on to win in Catalonia at the general election expected on 20 December, “that will serve as a second plebiscite that will send an extraordinary message to Europe and the rest of the world”.

This has been a year of shockwaves that have rattled Spain’s political establishment. The first came in May, when a series of leftist popular fronts swept to power in Madrid, Barcelona and other major Spanish cities. Hit by a series of corruption scandals, the established conservative and socialist parties have been losing ground to new forces: Ciudadanos on the centre-right and Podemos on the left. This is also true in Catalonia, except that the picture is coloured by the national question, which has further marginalised the established parties.

Polls show Catalans fairly evenly divided on the independence issue, with a small majority against. More significantly, there is little enthusiasm for secession in their capital, Barcelona, which in May voted in a leftwing mayor who’d stood on a platform of social issues rather than sovereignty. Few people believe it is a coincidence that Mas called the elections on what is a holiday weekend in Barcelona in the hope that many voters will be out of town. A low turnout in the capital would favour the secessionists.

Junts pel Sí are opposed by another popular front, Catalunya Sí que es Pot (Catalonia Yes We Can), made up of Podemos, the Greens and other leftists who are standing on social issues such as health and housing and are agnostic on independence. The other key runner is Ciutadans, the Catalan version of Ciudadanos, which is expected to pick up a lot of votes from those who recoil from the idea of independence. “Unilaterally declaring independence with or without a majority of seats is anti-democratic and amounts to a coup d’état,” says Ciutadans candidate Susana Beltrán.In propaganda terms Junts pel Sí, which is essentially the Catalan government plus a few allies, has the upper hand. It can count on a docile, heavily subsidised media to get its message across. Neither Ciutadans nor Catalunya Sí que es Pot can expect even a fraction of that exposure.

By presenting the poll as a referendum on independence the secessionists are also taking a risk. It may well incite people in the so-called “red beltway” of Barcelona, a string of overspill towns dominated by socialist voters of Spanish descent who are generally hostile to independence, to vote in greater numbers than they usually do in regional elections. On the other hand, Madrid’s intransigence continues to drive many moderates into the independence camp.