Explainer: Crunch time for Spain as regional, EU elections loom

By Joan Faus and Ingrid Melander

MADRID (Reuters) - Three weeks after a national election, Spain is still without a government. On Sunday it holds local, regional and European ballots that will go a long way toward establishing the country's political identity for the coming years.

Here's what's at stake:

WHY IS THIS A CRUNCH MOMENT？

Acting Prime Minister Pedro Sanchez's Socialists won the April 28 parliamentary election but failed to secure a majority.

After weeks of posturing on all sides, he can start discussing alliances in earnest once the results of Sunday's elections - widely viewed as a follow-up round to April's vote - are known.

But whatever happens, it won't be quick.

With the political landscape increasingly fragmented between five main national parties and several regional ones, negotiations seem certain to be complex, and likely to include regional or local deals in exchange for backing at national level or in other regions.

SO WHEN WILL SPAIN HAVE A GOVERNMENT？

That won't be before late June or, more likely, early July, the acting government's spokeswoman has said.

To get reelected, Sanchez will need the support in parliament of other leftist parties, Basque nationalists and the vote or abstention of at least one more lawmaker, possibly a Catalan separatist.

Before that, parliament will convene on Tuesday for the first time since April 28 to vote on speakers for both houses. In a likely foretaste of the broader consensus-building exercise, Sanchez has nominated two Catalan Socialists who favor dialogue with the separatists but oppose the region's independence drive.

Sanchez says he wants to govern alone with only ad-hoc support from other parties, but a variety of scenarios are possible.

WHAT'S AT STAKE FOR THE LEFT AND THE RIGHT？

On Sunday, 12 of Spain's 17 regions and all of its 8,000-plus municipalities will be up for grabs.

Opinion polls suggest the outcomes in the three most closely watched ballots - the Madrid region and the Madrid and Barcelona municipalities - are unclear.

Sanchez's party, seen getting the biggest number of seats in the simultaneous European Parliament election, is also aiming to reinforce its April 28 win on the domestic front.

The conservative People's Party (PP), which suffered heavy losses last month, is focusing on damage limitation and maintaining strongholds including the Madrid region.

Far-left Podemos, which says it want to govern in coalition with the Socialists, is also looking to shore up its support base after losing ground in the national election.

Center-right Ciudadanos wants to keep growing.

Far-right Vox, which for the first time got lawmakers into parliament with 10% of last month's vote, wants to be a kingmaker in regions, replicating the deal it struck last year with right-wing parties in Andalusia, Spain's most populous region.

Polls suggest Vox could be instrumental in determining who governs in Madrid and its surrounding region.

WHAT ABOUT CATALONIA？

The northeast region's bid for independence was a central theme of a divisive national election campaign, and it remains potentially pivotal for Sanchez's ambitions of forming a stable government.

Five jailed Catalan separatist leaders elected last month - four to the lower house and one to the senate - will be sworn in as lawmakers on Tuesday. [L5N22W1Z3]

But they remain on trial for their role in the region's failed 2017 independence bid and it is unclear what will happen thereafter.

Once sworn in, their rights as lawmakers could be suspended. The Supreme Court has recommended this but left it to parliament's governing body to decide on Tuesday.

If they are suspended but decide to keep their seats, the number of voting lawmakers in the lower house will fall. Sanchez could then be re-elected prime minister without the backing of the Catalan separatists.

One of the five, Oriol Junqueras, has said he will give up his national seat if he is elected to the European Parliament on Sunday.