Which president would most likely support the secession of the Southern states from the Union?

The president most likely to support the secession of the Southern states from the Union would be John C. Calhoun, who served as Vice President and was a strong proponent of states' rights and nullification. While not a president himself, his political ideology aligned closely with the interests of Southern states seeking to secede. Among actual presidents, Andrew Johnson, who succeeded Abraham Lincoln, had a more lenient approach to Reconstruction and might have been more sympathetic to Southern grievances. However, it is important to note that no president openly supported secession, as it was a constitutional crisis.