In March 1944, Cain was assigned to SHAEF headquarters in London, England where he directed the psychological warfare and public relations division of the G-5 Civil Affairs staff. Promoted to Lieutenant Colonel, he worked with many famous writers and journalists including Archibald MacLeish and Edward R. Murrow. In April 1944, Cain was approached by political supporters in Washington state to run as a Republican for the open U.S. Senate seat in the 1944 election. Although Cain was unable to campaign actively, he won the Republican primary and faced popular Democratic Representative Warren G. Magnuson in the general election. Cain ran a respectable campaign, but fell behind in the final weeks of the campaign, losing to Magnuson by 88,000 votes.