Probably the most famous Socialist in America during the early 20th century—besides Debs himself—was the writer Upton Sinclair.

Sinclair's novel, The Jungle, dealt with the lives of meat-packing workers and the disgusting conditions under which they worked. The Jungle created a national sensation, but not for the reasons Sinclair intended—he hoped his readers would so identify with the novel's hero, who becomes a Socialist, that they would be converted to Socialist philosophy.

Instead, readers focused on the unsanitary working conditions and revolting, unhygienic quality of American meat. The book led to a huge public outcry, very immediate reforms, an invitation to the White House to discuss meat matters with President Roosevelt, and the creation of USDA standards. As Sinclair wryly noted about his book,

I aimed for their [readers] hearts and hit their stomachs. (Source)

But whatever his plan, the popularity of The Jungle made Upton Sinclair a major voice for American Socialism.

Upton and Eugene (yes; we're on a first name basis with these guys) saw the world a wee bit differently, however. When WWI began and, even more, when America entered the war, Sinclair saw the Germans as a serious threat to America and believed America should join the Allies.

He advocated an idea he called "democratic defense" and thought that the government should take over all munitions factories and run them on Socialist lines. And he thought that a universal draft could lead to solidarity among soldiers, a Socialist ideal.

Basically, Sinclair didn't see the war as just a capitalist war about markets and he believed that the Socialists should support the government. He wrote to Debs and tried to convince him about this idea of defending democracy, but Debs wasn't buying it, feeling that the United States was acting far from democratically as it pursued its path to war.

Though Sinclair would ultimately resign from the Socialist party over this split, he still worked diligently to try to win Debs' freedom when he was imprisoned under the Espionage Act.