"Roosevelt made the progressive era possible," writes Joshua Hawley in Theodore Roosevelt: Preacher of Righteousness (Yale Univ. Press, 2008). "His spirit became its ethos. His politics of virtue, his warrior republicanism, was a gauntlet thrown down to an entire country and to himself, a challenge to be better, to be more, to be righteous. It was a challenge that defined his life and his time."
The word "overstatement" comes to mind when reading this. Yes, TR was influential. But crediting him with generating the progressive era's ethos seems a bit much. Alas, I have not read the book and will reserve final judgment until then. Meanwhile, I'm eager to see how Hawley (a former clerk for Chief Justice John G. Roberts) notes the influences of the social gospel and muscular Christianity on TR. George Will's description of the book suggests these ideas were quite prominent in his ideological foundation.
Roosevelt was an individualist who considered the individualism of others an impediment to the social unity required for national greatness. Having read Darwin's The Origin of Species at age 14 and having strenuously transforme himself from an asthmatic child into a robust adult, he advocated "warrior republicanism" (Hawley's phrase). TR saw virtue emerging from struggle, especially violent struggle, between nations and between the "Anglo-Saxon" race and lesser races. Blending "muscular Christianity," the "social gospel" -- which sanctified the state as an instrument of moral reclamation -- and Darwinian theory, TR believed that human nature evolved toward improvement through conflict.