Among the possible victims of circumstance in this winter of 1863 were the Democrats who made up a majority of the Indiana legislature. Without realizing it, these men were struggling against the fact that the American political system, wide enough for many things, had not by the founding fathers been made wide enough to contain a civil war. They were Democrats taking normal advantage of the fact that they had won an election, and what they were running into was the fact that there was no way, in this moment of all-out war, by which they could do that and nothing more. They wanted to oppose the party that was running the war, and in spite of themselves they could do no less than oppose the war itself. There could be no delicate shadings of action or belief. The administration was fighting for complete victory; to stand against the administration in the ordinary way, using the grips, feints, and arm locks of normal political struggling, meant in actual practice to stand for something less than victory--something a good deal less, perhaps, if the wrestling got really strenuous, so that the struggle might finally appear to be a struggle against the war itself rather than simply against the people who were conducting the war.
I don't think Catton's analysis goes quite deep enough here. The reason the Democrats couldn't oppose Lincoln without opposing the war is because they had a huge bloc of voters in the North who were pro-South, pro-slavery and hence, anti-war. If they criticized the handling of the war, they provided ammunition to the anti-war camp and found themselves tarred with the same brush. If they tried to match the militancy of the Republicans, then they would lose the support of a large bloc of Democratic voters. To sit in the middle was to please no one.
If there had been no significant opposition to the war in the North, then the debate could have been framed in terms of how to conduct the war, rather than whether to conduct it. The same dynamic is in operation now. The Democrats have been forced by their most radical element into an anti-war position. They now have no chance to win unless they can convince the American people that it is better to withdraw from Iraq than to stay until we achieve victory. That makes them the party of defeat. It is not an enviable position.