"Lots of groups ask their supporters to write to candidates during election campaigns, but the manifestos have already been written by then," he said in advance.
He added that charities should carefully consider which actions would make a difference. "Waving banners in the street makes you feel good, but doesn't move the needle," he said.
Supporters should visit their MPs rather than sending letters, he added. "This means you don't have total control of the message, but you will get depth and breadth of engagement."