MADRID (Reuters) -Spaniards will interrupt their summer holidays to vote for a new parliament on July 23, after an election campaign in which no one party has established a decisive lead and featuring irate exchanges between the main candidates for prime minister.

Polls show the conservative opposition People's Party (PP) of Alberto Nunez Feijoo beating Pedro Sanchez's ruling Socialists (PSOE), but failing to secure an absolute parliamentary majority.

To secure a majority of the 350 seats that it needs to form a government, the PP would almost certainly have to ally with anti-immigration, anti-feminist Vox, a move that could give the far-right a role in government for the first time since the end of Francisco Franco's dictatorship in 1975.

WHY WAS THE ELECTION CALLED?

A national elections was due by December, but Sanchez unexpectedly called a snap election after the left did badly in local elections in May.

The move was an apparent attempt to wrongfoot the PP, forcing it to campaign while also negotiating uncomfortable post-local ballot coalition deals with Vox.

WHO IS IN THE LEAD IN THE POLLS?

Polls suggest the PP will win just over a third of votes and the PSOE just under a third. Potential kingmakers Vox, and popular Deputy Prime Minister Yolanda's Diaz's leftist bloc, Sumar, are both tipped to win 10% and 9% respectively.

As the PSOE hoped, the PP's popularity was slightly dented by its negotiations with Vox, some of whose extremist views were incorporated into the coalition agreements between the two right-of-centre parties.

Sumar meanwhile sealed an alliance with leftist parties and gained ground.

However, about 54% of voters thought Feijoo won an ill-tempered live television debate with Sanchez, against 46% for the prime minister, according to a Sigma Dos survey.

WHAT HAPPENS ON THE DAY?

Polls open at 9 am, with 37.4 million Spaniards are registered to vote, including 2.3 million abroad and 1.6 million for the first time. At least 2.5 million have registered to cast ballots by post.

A total of 350 lower house deputies and 208 senators will be chosen. Party lists for lawmakers are closed, so voters pick a party rather than a specific candidate, but chose up to three regional senators.

Polls close at 8pm, except in Canary Islands which are an hour behind, with the winning party expected to be announced before midnight.

WHEN WILL A NEW GOVERNMENT BE FORMED?

New parliament must be constituted by August 17, when the oldest lawmaker is nominated temporary speaker. Lawmakers vote the same day to pick a permanent speaker, an event that indicates how blocs are aligning and their relative strength.

King Felipe VI will begin meeting party leaders soon after to hear their pitches, and must then put forward a candidate for prime minister.

There is no time limit for the candidate's negotiations to form a government.

If the candidate fails to obtain an absolute majority in a parliamentary vote, a re-run requiring only a simple majority is held 48 hours later. If they lose again, the king has to pick a new candidate.

COULD THERE BE ANOTHER ELECTION?

If no candidate secures a majority within two months of the first vote, new elections have to be called.

Spain has held four in the past four years because coalition agreements could not be reached, with some such impasses lasting as long as six months.

Analysts agree that this time, if the PP and Vox cannot cut a deal, further elections are a distinct possibility.

ANY OTHER POTENTIAL ISSUES?

The vote will take place at the height of summer, with temperatures likely to top 40 degrees C in parts of the south.

With millions of Spaniards on holiday and more voting by post than ever, unions and the PP have expressed concerns that some risk being disenfranchised due to administrative bottlenecks.

Voter participation has declined from 80% in the election after Franco's death, to 66% in November 2019. This time, voter apathy coupled with holidays and hot weather could dent turnout further.

(Reporting by Aislinn Laing, additional reporting by Belen Carreno, graphics by Andrei Khalip; editing by John Stonestreet)