Lewis Mitchell is first mentioned as being a senator from Westsylvania, nominated to be VP of Edward Mahan in 1888. He is dropped from the Mahan ticket in 1892, because of his support for a filibuster going on in Hawaii at the time.
The thing in Decades of Darkness that he is most known for is the North American War. Despite the fact that he apparently wanted a war with Britain at all costs, he knows the problems, and is able to get Germany and Nippon to stand down for the NAW. He even offered a peace midway through that asked for nothing in return but Guyana and British Honduras. But of course it was rejected.
So the Treaty of Washington at the end of the war was a lot harsher, but not as bad as it could have been, with New England conceding some of New Jersey's southern counties, and not Michigan or all of Jersey as some people wanted. However, the victory also meant that *America now was free to conquer Britain's former allies Costa Rica, Colombia and Venezuela.
All seemed poised for Mitchell to run for a third term. However, it was discovered that his wife was bisexual, so shamed by the scandal, he withdrew.