In the various polls of historians ranking the presidents, Buchanan is usually down toward the bottom -- among the worst, typically in the company of Pierce and Harding and sometimes Andrew Johnson and Grant. The Republic has survived them all, just barely in the case of Buchanan, but what makes him a bad president?
According to Tulane University's "Crisis at Fort Sumter" series, "Buchanan never expressed regrets for any of his public acts, and he predicted that history would vindicate his memory. But while he has been credited with good intentions in his efforts to avert civil war and achieve a compromise, his general handling of the sectional crisis has been criticized.
"The reasons vary. Some scholars find fault in Buchanan's character. They portray Buchanan as timid, weak, and indecisive. Insecure and excessively reliant on the opinions of others, he was also prone at times to adhere stubbornly to a decision, however flawed. Others emphasize Buchanan's reliance on legalistic thinking and the power of reason, which hampered his ability to deal with the intense passions and emotions that infused sectional politics. Still others claim that Buchanan was unable to comprehend the nature of the sectional dispute over slavery. He never gave to the North's concerns about slavery and southern power the same sympathy and understanding that he gave to the South's complaints."
The entire essay is here.