Spaniards have become increasingly disenchanted with politics as unemployment, fraud and separatist tensions in Catalonia cloud a nascent economic recovery.
This has taken its toll on Prime Minister Mariano Rajoy's government, which has also lost enthusiasm from traditional conservative voters since it withdrew a controversial abortion law last month.
Rajoy's People's Party (PP) would win 15.9 percent of the vote if elections were held now, about half of what it won in 2011, the Metroscopia poll published in newspaper El Pais showed.
While support for Spain's opposition Socialists has ticked up to 20.7 percent versus 20.3 in the same period, new leftist party Podemos has emerged as the country's third largest political force, gaining 14.3 percent of votes, the poll showed.
Led by university professor Pablo Iglesias, Podemos has plugged into discontent with big establishment parties, threatening an end to a two-party political system that has dominated Spain since its return to democracy in the 1970s.
Together, the PP and the Socialist parties won 52 percent of the vote in 2011, at the height of a financial crisis that was threatening to push Spain into an international bailout, but today would only capture about 37 percent, the poll showed.
Spain's next general elections is due late next year.