It is unusual for a high-profile journalist to play a central role in a political party in the midst of an election campaign, but that is just what Paul Mason, former Guardian writer, former economics editor for Channel 4, has been doing.  In February 2016, he quit Channel 4 so he could participate fully in the affairs of the political left without the presupposition of impartiality that is (unevenly) imposed on broadcasters.

And participate he has: since joining Momentum, the pressure group formed in support of Jeremy Corbyn’s policies after his surprise 2015 win of Labour’s leadership contest, his writing has been explicitly partisan. He has authored emails to Momentum members exhorting us to get involved in election campaigning, and he has spoken out on Twitter and his own Medium blog in support of Corbyn’s Labour, becoming a leading voice of the unprecedented movement that has swelled the party’s ranks.

Mason has been traveling the country as a sort of embedded journalist-activist, visiting the marginal constituencies — those parliamentary seats where, in 2015, Labour had only won or lost by a few thousand, or a few hundred, votes. On Saturday, he came to Plymouth, where I live.