Richard Harding Davis 
The Consul [PDF ebook] 

The Consul has a sad side to it. The main character, Marshall, was once Abraham Lincoln's right hand man, but since the President's death, Marshall has fallen out of political favor. After many years in the navy rising to admiral and many more years as an ambassador to a myriad of countries, he has been "marooned" in Columbia. Forgotten. His hometown grew up, he has few friends, and he simply has no connection to his old life until the plot of the story: Senator Hanley pays him a visit, and on that ship is also an admiral that's a friend of Marshall's.
What will happen? Will party boy Hanley destroy Marshall's career? Will Admiral Hardy find a way to intervene?
It looks good. Marshall stands up for righteousness. But Hanley makes threats.
The plot twists left and right until the end.