In further action upon his political principles, Macdonald signed his name to the "Writers and Editors War Tax Protest" by which he refused to pay income tax to undermine the financing of the undeclared Vietnam War. Likewise, along with the American public intellectuals Mitchell Goodman, Henry Braun, Denise Levertov, Noam Chomsky and William Sloane Coffin, Macdonald signed the antiwar manifesto "A Call to Resist Illegitimate Authority" (12 October 1967) and was a member of RESIST, a non-profit organization for co-ordinating grass-roots political work.