According to wikipedia...
So, what if he had run? Could he have united the opposition to Lincoln behind him? How might he have fared in the election? If he had won, he, rather than Grover Cleveland, might be the only President to hold two non-consecutive terms, which would be interesting in and of itself.
After losing the Democratic nomination for reelection in 1856, [outgoing President Franklin] Pierce retired and traveled with his wife overseas. He returned to the U.S. in 1859 in time to comment on the growing sectional crisis between the South and the North, often criticizing Northern abolitionists for encouraging ugly feelings between the two sections. In 1860 many Democrats viewed Pierce as a solid compromise choice for the presidential nomination, uniting both Northern and Southern wings of the party, but Pierce declined to run.
So, what if he had run? Could he have united the opposition to Lincoln behind him? How might he have fared in the election? If he had won, he, rather than Grover Cleveland, might be the only President to hold two non-consecutive terms, which would be interesting in and of itself.