Gentlemen, you never heard of the National Union Party which Lincoln and his friends started after the Republican party? As I understand it, basically he was sucking up to the Northern Democrats, trying to compromise with them and some of his Republican friends, only the Republicans started leaving and going back to the G.O.P. so eventually it became just another branch of the Democratic party in new clothes. The last congressman left the N.U.P. in 1867.
You can read about this if you scroll down under Lincoln's photo and click on the word "National Union" on the right side of this article, just a few lines above Lincoln's signature under Political Party:
http://en.wikipedia.org/wiki/Abraham_Lincoln