In the summer of 1936, Bethune was in Toronto to raise money to send medical aid to the Republican army in the Spanish Civil War. He, Fritz Brandtner (1896–1969), and Pegi Nicol (1904–1949) crashed a dinner party at the Clarks’ one evening—with profound implications for the artist. Up to that point, she had not been politically minded, but Bethune, who had recently become a Communist and had visited Leningrad the year before, likely encouraged her to become actively involved in the Spanish Republican cause.

As a result of meeting the charismatic Bethune, with whom she had a brief affair, Clark embarked on works based on her memories of Russia. She also exhibited her first work with political content, Presents from Madrid, 1937, a watercolour depicting the small mementoes from the war that Bethune sent her in correspondence from Spain. In April, she wrote an article in the left-wing journal New Frontier advocating a role for the artist in society and countering a piece by Elizabeth Wyn Wood (1903–1966), published earlier in Canadian Forum, arguing that artists could ignore social and political affairs.

In the summer of 1937, Clark produced her second painting with political content, Petroushka, 1937. She considered this canvas to be her most important work and sent it to the Great Lakes Exhibition (1938–39) and the CGP section of the New York World’s Fair (1939–40), where it would be seen by larger American audiences. Petroushka is a piece of strong social commentary whose source is veiled. It brings together Clark’s childhood memories of puppet theatre in the streets of Saint Petersburg and her response to a violent clash between police and striking steelworkers in Chicago, in which five workers were killed.

Clark taped the article (“Five Steel Strikers Killed in Clash with Chicago Police,” Toronto Daily Star, June 1, 1937) to the back of the canvas to document the source. She transformed the story of Petroushka (the Russian equivalent of Punch or Pulchinello) to describe the plight of the worker, subjugated by the interests of capitalism (the banker) and its enforcers (the policeman). The viewer looks down on the scene in a cobblestoned yard between apartment blocks. The crowd responds to the performance with catcalls and clenched fists—an anti-fascist symbol of unity, strength, and resistance used here to indicate the artist’s support for their cause. Two preparatory sketches, including Study for Petroushka, 1937, illustrate the evolution of the painting.

In the closing years of the decade, Clark worked for the Committee to Aid Spanish Democracy and tried (unsuccessfully) to bring Pablo Picasso’s (1881–1973) Guernica, 1937, to Toronto in 1939 as a fundraising initiative to assist Spanish refugees. Clark did not produce a large number of paintings with social purpose during her career, yet they are her best-known works today.