Buchanan was a very poor president, but so were the series of presidents from Polk to him. The election of 1844 was a victory for Manifest Destiny and the Expansion of Slavery, first into Texas and then into the territories. It established that, instead of gradually and peacefully ending slavery, the preservation of slavery and subordination of freedom in our country would be the cornerstone position of the Democrats for the foreseeable future. The Whigs could not confront this pro-slavery position head-on, and just come out and say slavery was evil. Therefore, that party had to fall apart, and eventually be replaced by one that had the courage to address the issue.
With regard to Warren G. Harding, he was very much like Bill Clinton. He was a very personable fellow, charming, good natured, and prone to indulgence. He did not have the strength of moral character we should demand in our public leaders, and his administration was somewhat corrupt. On the other hand, he had good policies and our country prospered. The main difference between Harding and Clinton is that Clinton was impeached for his betrayal of the trust of the American people, while Harding had the good sense to die while in office and spare us the anguish of impeaching him.
It can be argued that Harding and Buchanan were inept, but passive, and therefore the damage caused was self limiting. Carter, on the other hand, aggressively pursued policies that were directly detrimental. In 4 short years he cause a lot of damage, IMO (lost Iran, Afghanistan, loose money then tight, stagflation, high interst rates, inflation, malaise) A very busy, self important incompetent.
Some of the Whigs were slaveholders (e.g. Zachary Taylor), so they couldn't take an unambiguous anti-slavery stand. The irony is that if the Liberty Party (anti-slavery) had not run a third-party effort in 1844, Clay would have won, and all of American history would have been different. Without the Mexican War and the territories acquired in 1848, the issue of slavery in the territories wouldn't have torn the country apart in the 1850s as it did.
Unlike Clinton, Harding was not a sociopath, and he doesn't seem to have been personally guilty of corruption--he just had bad judgment picking people who betrayed his trust. He didn't use the power of the federal government to destroy people who were investigating corruption and wrongdoing.