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Spain's political parties are set to start campaigning for the country's April 28 general election, in which the ruling Socialists are trying to remain in power and the extreme right is seeking to enter Parliament.

The campaign period that starts at midnight on Thursday promises to be a fierce fight for what polls show is a large pool of undecided voters. Successfully wooing those voters could be key to Spanish Prime Minister Pedro Sánchez continuing to govern or ushering in a coalition government made up of right-wing parties.

Polls indicate the far-right party Vox is poised to win its first seats in the lower house of Spain's parliament. If that happens, it could increase the fragmentation in Spanish politics by introducing a fifth national party of significant size.