A Metroscopia poll for El País published on Sunday suggested both the furthest right and furthest left options available to Spanish voters have received a boost following the decision by Podemos and United Left to present a joint list at the general election on June 26.
The paper's editorial criticised the results as "irresponsible" polarisation.
Mariano Rajoy's Popular Party increased its estimated share of the vote to 29.9%, rising to a post-election high following the low in February at 24%. At the last election, the PP won 28.7% of the vote.
Unidos Podemos ("United We Can"), the new Podemos-United Left list, was scored at 23.2% in its first joint showing, three full points clear of the Spanish Socialist Party (PSOE) on 20.2%.
Ciudadanos fell further to 15.5%.
The two parties who made the only attempt to appoint a new Prime Minister during the spring—the PSOE and Ciudadanos—have both been trending downwards as the PP rises.
Sunday's Metroscopia joins El Español's forecast in predicting that Unidos Podemos will overtake the PSOE to become the leading party on the Spanish left.