Could the POD be on April 14th, 1865 itself? If the Booth conspiracy fails to assassinate Lincoln, Johnson, Seward, and Grant its not an attempted coup, there is no state of emergency, and maybe the government doesn't look too hard into the links between the conspirators and various Democratic politicians. With no period of one party Republican rule, inertia and the logic of the two party system will re-assert themselves. Even if one or two of the targets get nailed, as long as it is not Lincoln, I think alot of that stuff would be swept under the rug.