Though it can be said with reasonable assurance that these men helped shape modern America it can also be said that they were unscrupulous doing it. Workers were badly treated and striking was usually broken up with force. Unions were outlawed under the Taft administration, a law which Roosevelt upheld.
Roosevelt sided with the unions during his first term as president. Taft continued most of Roosevelt's domestic policies after Roosevelt left. Nether of them would be willing to outlaw unions.
To many in the American workforce it became clear that there was no justice in the capitalist system and many were disillusioned with their rights. Though the majority continued to support the elections process many were turned to the socialist camp with the outlawing of unions. Socialists would become a force to be reckoned with in the coming years.
And this is why Roosevelt sided with the unions. Theodore Roosevelt was at heart a Populist, and He realized that siding with the unions would destroy most of the Socialists propaganda.
In response to the assassination of President McKinley many Socialists take less extreme methods of demonstration to heart. They actively protest peacefully and attempt to use democratic methods to gain power. This brings them into conflict with many hardcore Marxists who believe revolution is the only way to power. This was the beginning of the quiet split of American Socialism.
President McKinley was assassinated by an Anarchist wannabe. I don't think it had much effect on American Socialists.
Republicans still had a majority on the vote as they had Roosevelt, America’s hero. Having stood up to the European powers and established good relations with their premier European ally Germany. Many on the Socialist or Democratic ticket however, resented Germany for having made the harsh statements that lead them to nearly the brink of war with Britain. This in no way hindered America’s support for Germany in the Great War however, as any dissent was drowned in the voice of popular demand. Many American volunteers even began to filter over to Germany in order to fight against their hated European enemies. They helped establish two divisions in the Kaisers armies (respectively known as the Kaisers American Rifles). These militaristic trends would be rather ominous in the years to come.
You do realize this means the Kaiser will win, right?