Socialism is said to have formed the "left wing of Progressivism." Progressives shared some of the same concerns that the socialists had. For example, both of the groups felt that big businesses had come to be too strong and were dominating government while doing harm to the working classes. Both wanted to rein in big business as a result.
But most Progressives rejected the remedies that socialists would have used to fix this. They were willing to accept things like government ownership of necessary utilities, but they were not willing to go any farther than that. Instead, they wanted to reform the capitalist system rather than to overthrow it. In a way, the Progressive reforms sprang up in an attempt to save capitalism and democracy by reforming it so that socialism would not gain traction in its attempt to overthrow the system.