In the crucial Presidential election of 1864, with the nation at war with itself and his own party deeply divided, Abraham Lincoln decided to split the ticket. Faced with the disaffection of the Radical Republicans and the anticipated popularity of his Democratic rival, General George McClellan, with the all-important soldier vote, Lincoln ran for reelection not as a Republican but as the National Union Party candidate. He selected the military governor of Tennessee, "War Democrat" Andrew Johnson, as his Vice President. Some historians consider this choice of running mate to have been his greatest blunder, and indeed the failed Presidency of Andrew Johnson is generally regarded as one of the very worst in US history.
There has been an animated discussion over at Kevin Levin's Civil War Memory concerning alternative choices that Lincoln could have made, keeping within the political parameters that brought about the National Union Party and were in play at thetime, and whether or not the short-term outcome of a different presidential successor than Johnson would have made much difference. This sort of counterfactual analysis is mother's milk to me, and having put in my two cents in Kevin's forum I reserve the right to expand and extend my remarks for the record in my own.
One of the many perils of counterfactual conjecture is that every alteration to historical events acts upon all those that follow, with the potential to expand exponentially into an unrecognizable reality. This is why time travel in science fiction novels presents such hazards to those characters who attempt it and thus provides a reliable plot device. One of the first rules of counterfactual hypothesis is to make as few changes to the conditions leading up to the alternate version of history as possible. It is all about the "want of a nail", rather than in this case the lack of an industrial economy in the southern states, or the absence of a confederate air force.
There is also the possibility that 2nd order counterfactuals stemming from the first might well bring about the same historic outcome from a different direction. This lies at the very heart of the question of whether Lincoln could have made a better choice than Johnson, and if that would have lead to a meaningfully different outcome during his successor's presidency than what actually occurred. Sometimes, no matter where they begin, all roads must inexorably lead to Rome.
Lincoln could not run as the National Union Party candidate - a party made up of moderate Republicans and hawkish Democrats - without a running mate who would appeal to key voting blocks. Then, as now, the electoral votes were the critical factor. Lincoln needed to win a number of large states with many electors to secure reelection, as well as secure the support of the war Democrats.
The top five states with the most electors were New York, Pennsylvania, Ohio, Illinois and Indiana, and several of these states were peace Democrat strongholds. To counter Little Mac and secure more of the soldier vote, Lincoln might have turned to a War Democrat from the Midwest, such as any of several of the Fighting McCooks .
A McCook presidency has too many variables to project with any confidence what the course of Reconstruction might have been. Assuming any of the “Tribe of Dan” or “Tribe of John” agreed to run on the national Union Party ticket, they would have faced tremendous challenges from the Radical Republicans. Even if this split ticket won the election, Lincoln’s assassination might have lead to a very weak McCook presidency with a hostile congress and pressure from northern Democrats to go easy on Reconstruction and light on the rights of freedmen. Perhaps a President McCook would become even more of a hardliner (in for a penny, in for a pound). Or perhaps he would have reshuffled the cabinet, pushing some of the radicals out. There might have been no Seward’s Folly: no Alaska. Although several of the McCooks when to to political careers after the war, as a successor to Lincoln they would certainly be no better than Johnson.