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Four runner-ups to become the next Spanish prime minister are exchanging attacks while slipping in campaign pledges during the first of two televised live debates ahead of Sunday's general election.

Monday night's debate on Spanish public television and the second on Tuesday on a private broadcaster are seen as key in mobilizing nearly one-third of voters who polls say remain undecided.

Socialist leader Pedro Sánchez is leading voting predictions in his bid to stay in office, though short of a majority to form a government alone. He was the target of most of the criticism from opposition candidates in the debate.

A surging far-right party was left out of the debate after Spain's electoral board disallowed a five-way debate, ruling that other smaller parties would also need to be invited.