BARCELONA (Reuters) - Four Catalan leaders on trial over a 2017 bid to split their region from Spain have said separatists should be more flexible about entering negotiations on forming the next Madrid government after a April 28 national election.

Spain's Socialist leader and current Prime Minister Pedro Sanchez delivers his speech during a PSOE party meeting before he kicks off his political campaign ahead of the April 28 general election in Dos Hermanas, near Seville, Spain April 11, 2019. REUTERS/Jon Nazca

Socialist Prime Minister Pedro Sanchez is forecast to win the most seats in the vote, but could need Catalan separatists’ backing to form a government. A unionist, rightist coalition winning a majority is another possible scenario.

The Catalan leaders’ call, made in a letter in Saturday’s edition of La Vanguardia newspaper, said separatists should enter talks with potential coalition partners as long as they refused to rule out an independence referendum as a “possible solution” for the region.

That marked a softening of their previous stance and raises the possibility of compromise on an issue that has vexed past coalition talks.

“If it depends on us, we won’t look the other way when it is time to form a stable government, provided the candidate commits to dialogue and doesn’t rule out an independence referendum as one possible solution,” the leaders’ letter said.

They had said previously that holding an independence referendum for the region would be a non-negotiable condition for entering discussions to form a national government. Madrid has categorically ruled out holding such a referendum.

While rejecting an independence ballot, Sanchez has taken a more conciliatory tone toward the restive region since coming to power last year.

He called a snap election in February after failing to win the support he needed from Catalan separatists to pass his budget.

The four authors of Saturday’s letter -- all members of former leader Carles Puigdemont’s party -- are among 12 Catalan politicians and activists being tried on charges including rebellion and misappropriation of funds for their role in organizing a 2017 referendum and subsequent failed declaration of independence.

They also sought to rally support for their party by saying a strong showing would lessen the chances of a right-wing coalition, including the far-right party Vox, being able to form a government.

Despite narrowly winning a majority in 2017 snap elections called in the wake of the secession crisis, Catalan separatist parties have struggled to further the independence movement in the face of successive Madrid governments unwilling to negotiate any split from Spain.