This link: http://agendapublica.elperiodico.com/por-una-nueva-cultura-fiscal/
It includes references to studies in Spain on public opinion and on the willingness of Spaniards to withstand a higher tax burden.
The origin of the data and the applied methodology that may serve you comes from the INE (National Statistics Institute of Spain), specifically "Public Opinion and Fiscal Policy 'of the CIS from 2003 to 2017"