BARCELONA (Reuters) - Catalans vote on Sunday in a regional election that could determine the stability of Spain's Socialist-led government and also gauge the strength of a pro-independence movement that has roiled the country for a decade.

A government led by Prime Minister Pedro Sanchez's Socialist party in Catalonia after a decade of separatist governments would usher in a new era, said political scientist Toni Rodon.

"If the separatists lose their absolute majority in parliament, it would definitely represent a change of cycle," Rodon said. "Whether this is temporary or long-term, we shall see."

But a win for the Junts party would bolster the separatist cause and create further predicaments for Madrid. Junts' candidate is Carles Puigdemont, who was Catalonia's president during an ill-fated attempt to wrest the region from Spain in 2017, and he has vowed to resurrect an independence bid.

Since taking office for a second term last November, Sanchez has relied on a fragile alliance with separatists to pass legislation in the national parliament, earning the ire of conservative opponents.

Puigdemont, who faced prosecution in Spain over the failed independence bid and has been living in self-exile but who is set to return home soon thanks to an expected amnesty, has warned that Junts might withdraw its support if the next Catalan government is one he cannot accept.

Opinion polls forecast a comfortable lead for Socialist candidate Salvador Illa in the election, ahead of Junts and its more moderate rival Esquerra Republicana de Catalunya (ERC), which currently governs the wealthy northeastern region.

The Socialists stand to win 40 seats with Junts getting 34 seats and ERC 26 seats, according to a poll of polls by El Pais newspaper.

But some surveys also show around 40% of voters are undecided, so any outcome is possible.

Illa has focused his campaign on pledges to revive the region's economy, which since the independence drive has seen its economy fall behind Madrid's.

"Catalonia has been performing way below its capacities in the past 10 years," Illa said earlier this week in a televised debate, saying he would develop clean energy.

CONCILIATORY APPROACH

He defended the Socialists' conciliatory approach with separatists seven years after they ignored a court ban and held an independence referendum, triggering Spain's worst political crisis in decades.

Puigdemont fled to Belgium in 2017 to escape prosecution. But he plans to return to Catalonia thanks to an amnesty bill put forward by the governing Socialists that would annul the arrest warrant he faces, once it comes into force in late May or June.

Puigdemont said this week he was the best candidate to "protect and defend Catalans' interests".

It was a sentiment shared by Roser Aguilar, 69, who traveled to a rally in the French town of Argeles-sur-Mer, near the Catalonia border, where Puigdemont is campaigning.

"I believe Puigdemont is the only one who can take us towards achieving independence," she said.

Any victor in Catalonia on Sunday will probably have to govern in coalition, since no party looks capable of reaching the 68-seat threshold for a majority on its own.

If the Socialists win they may seek an alliance with ERC or Junts, although both separatist parties have so far ruled that out.

Failing that, they could attempt an even more unlikely partnership with the conservative People's Party, their main rivals at a national level.

If they took power, Junts and ERC, would seek to lead a new pro-independence government. But some opinion surveys have forecast they would fall short of the seats required, including with the support of smaller separatist parties.

If post-election negotiations fail to produce an agreement by August, a repeat election would take place in October.

(Reporting by Joan Faus; Additional reporting by Nacho Doce; Editing by Charlie Devereux and Frances Kerry)

By Joan Faus