The Pony Express by Henry James Forman

When young gambler Jack Weston thwarts a senator's plot to rig the 1860 election in California, Weston must race against time and foes to ensure the truth arrives first. Battling Indians, rivals, and his own conflicted heart along the rugged Pony Express trail, Weston rides for justice and country, though each mile takes him farther from his beloved Molly.