 Cantilupe raised his voice against papal demands for money, and after the death of Grosseteste in 1253 was the chief spokesman of the nationalist clergy.At the parliament of Oxford in 1258 he was elected by the popular party as one of their representatives on the committee of twenty-four which undertook to reform the administration; from that time until the outbreak of civil war he was a man of mark in the councils of the baronial party.During the war he sided with Simon de Montfort, 6th Earl of Leicester and, through his nephew, Thomas de Cantilupe, who was then Chancellor of Oxford University, brought over the university to the popular side.