Here are some  sources that might be useful:

Philip S. Foner, Mark Twain: Social Critic (New York: International
Publishers, 1958)

Louis J. Budd, Mark Twain: Social Philosopher (Bloomington: Indiana
Univ. Prss, 1962).

Kay Moser McCord, "Mark Twain's Participation in Presidential
Politics," American Literary Realism 16:2 (Fall 1983): 262-271.

William M. Gibson, Theodore Roosevelt Among the Humorists: W. D.
Howells, Mark Twain, and Mr. Dooley (Knoxville, Univ. of Tennessee
Press, 1980).

Jim Zwick