(EN) Francesco Manetto, Could a pair of minority groups spell the end of Spain's two-party system?, in El País, 22 maggio 2013. URL consultato il 31 maggio 2015.
«And the main beneficiaries of the steady decline in voter support for the Socialist Party (PSOE) and the Popular Party (PP) are the leftist coalition United Left (IU) and the center party Union, Progress and Democracy (UPyD)»