True. The 1912 election is a centerpiece in history classes, but while the total number of votes increased from 1908, the percentage of voter turnout actually went down. If the statistics are right, a higher percentage of the population even turned out in 1904.
Of course the muckrakers and yellow journalists impacted the politics, but I just don't see it as defining of politics the way you do. The MSM of the day far more spoke to the rising middle class than to tenements, disease, and anarchists.
Certainly the prestige press like the Times wrote for the middle class, not for tenement dwellers and anarchists, but the situation may have been like today's: the people who really influence politics are elites and activists and those who are intensely interested in things that the general public isn't. I'm definitely no expert on the period, but my understanding is that leaders like Roosevelt and Wilson and intellectuals like Brooks Adams were very much captured by visions of ancient Rome and the French Revolution - plutocracy, decadence and anarchy.
To anxious patricians and professors, add the many religious believers inspired by the social gospel, and young intellectuals and social workers. Add to that the many businessmen who were somehow taken with TR's progressive movement. It wasn't a mass movement of the people who put Roosevelt and then Wilson in the White House, but once there, they were able to act on what they and others had been thinking and talking about.
I suspect people voted for Roosevelt and Wilson just because they considered them to be the rightful Republican and Democrat nominees (even when TR was running as a Progressive), but I have to wonder about the "drip-drip-drip" effect. People weren't crying out for reform, but if they heard about the Bradley Martin's decadent ball, or the Jungle or The Shame of the Cities or the Triangle Shirtwaist fire it might have made them more favorable to notions of reform. Not "the people," but enough people to affect the thinking of the day.
In my research of the 1907-1913 period, I read the NYTimes editions from every day (literally) and surveyed the Washington Star across the same. I also surveyed many of the major progressive periodicals, such as McClures, American Magazine, Collier's Pearson's and Scribner's, among others, as well as some of the more mainstream publications such as Munsey's or the Saturday Evening Post.
That is very commendable. Keep us informed as your work progresses.