A new [Sociométrica poll for El Español][poll] at the weekend suggested a drop of 40 seats for the governing Popular Party at a new general election. Mariano Rajoy's party currently holds 137 seats in Congress, but the new poll predicts only 97.

The biggest winner would be Ciudadanos, led by Albert Rivera, whose share of the total would rise from the party's current 32 seats up to 86, a rise of 54.

Such a large shift on the Spanish right would place three parties—the PP, Ciudadanos and the Spanish Socialist Party (PSOE)—on an approximately equal footing in parliament, with between 22....