Agreed with the above post. Sinclair believed that the American public would be so shocked and horrified over what they discovered in The Jungle that they would move away from capitalism altogether and embrace Sinclair's passion, his answer to all our ills: socialism.
When Sinclair got an audience with then President Teddy Roosevelt, he saw it as his shining moment, his chance to convince a sitting President to pursue socialist reform. Teddy wasn't having it. He listened politely, and he did pursue reform, but in the meatpacking and consumer protection areas. He did nothing to move us towards socialism, which deeply disappointed Sinclair.
Upton Sinclair wrote The Jungle in order to increase awareness of several social ills that he saw happening, and felt very strongly about. The book is the story of an immigrant family who comes to Chicago with high hopes of a better life. They settle in the area of the meat-packing plants, buy a house, and for a while things look good. But the immigrants are essentially used up like the animals they are processing--they are used up, every part, until there is nothing left, because there are plenty to take their place. Sinclair exposes the unsanitary practices of the meat-packing plants, the lack of nutrition in what they put out, and the horrible working conditions of the poor. The people are cheated in their housing, subjected to dangerous working conditions, not cared for when they become injured, and are treated as if they don't matter at all. Sinclair's book helped get legislation passed--the Pure Food and Drug Act, and the Meat Inspection Act.