Updike, through his protagonist, Alfred L. Clayton, wrote the following about Buchanan and the other "bad" Presidents leading up to the Civil War:
The challenge is, for the historian, to love the unlovable. . .He (Buchanan) tried to keep peace. That whole decade of Presidents did, Fillmore and Pierce and Buchanan--try, I mean--and they succeeded, they did keep the South placated, and in the Union, which was important, since if war had come in 1850 instead of 1860, the outcome might have been very different; the South had all its assets in place--the military tradition, the great officers, the down-home patriotism, King Cotton--and the North still needed to grow. And precious little thanks they've got from history for it--the doughface Presidents.
The term doughface was applied to northerners sympathizing with the south. Pierce has been accused of being pro-slavery, which is probably unfair. He was a Jacksonian Democrat and strict constructionist who believed the Constitution allowed slavery, but stated he personally was opposed to it.
Perhaps, as Updike's character suggests, we should love the unlovable and thank Fillmore, Pierce, and Buchanan for delaying war until the North was in a position to win. Interestingly, it was Jefferson Davis, Pierce's Secretary of War, who did much to build up the U.S. military before secession, when he became president of the Confedrate States of America.