The last election, in December, broke the mould of 40 years of stable conservative or Socialist majorities and failed to produce a government.
The last election, in December, broke the mould of 40 years of stable conservative or Socialist majorities and failed to produce a government, as upstart parties channelled growing resentment of the establishment following an economic crisis and a raft of corruption scandals.
Opinion polls suggest the parliament that emerges this time will be just as fragmented as the previous one. Four big parties and six smaller regional ones are likely to win seats in the 350-strong assembly, none of them coming close to a majority.
The centre-right People's Party (PP) looks set to be the biggest party again, with around 120 seats. But its natural coalition partner, the liberal Ciudadanos ("Citizens"), appears likely to win only about 40 seats, leaving them well short of the 176 needed for a majority.
In theory, the rise of Unidos Podemos ("Together We Can"), a leftist alliance led by Podemos, could offer a way out. The 90 seats it is expected to win, combined with around 80 for the Socialist Party (PSOE), would be close to a majority. Support from some of the regional parties could enable them to form a government.
Many analysts believe, however, that the 137-year-old Socialist Party would prefer to form a 'grand coalition' with the PP, led by the acting prime minister, Mariano Rajoy, or give passive support to a minority PP government, rather than combine with a group that threatens their existence.
"This is a crucial time for the left. Our time has come. We have an opportunity for change," said Carlos Martinez, a retired administrative clerk who cast his ballot for Unidos Podemos in the Arganzuela neighbourhood, in the south of Madrid.
However, the 77-year-old, who voted in December for the former communists of United Left, now part of Unidos Podemos, said the anti-austerity alliance might find it hard to govern because other parties may coalesce to block it.