An increasing number of Americans -- and not just conservatives -- are expressing the opinion that Barack Obama's presidency is as calamitous for the United States as was that of Jimmy Carter.  If so, then in the relatively short span of three decades, America has experienced two spectacularly flawed presidencies, each comparable to only a handful that have occurred over the life of the Republic.  Could these two recent tragedies have been predicted?  Are there any identifiable personality traits, past experiences, or political trajectories that might enable voters to foresee impending abject failure in a presidential candidate?

In order to respond to that query, we must ask: who were our worst presidents?  An immediate problem in addressing the question is that in most of the well-known surveys that rank our presidents, the overwhelming majority of the opinions were solicited from leftist historians and political science professors.  Thus, biased thinking skews the results.  For example, Calvin Coolidge is typically cast as a horrible president, while Woodrow Wilson is extolled as a great one.  It is said that Coolidge was absent while business ran roughshod over the working man, complicit in the rich virtually escaping federal taxation, and indifferent to the plight of American minorities.  Wilson was heralded for expanding the rights of women, modernizing America's antiquated constitutional structure, and improving the condition of the poor.

These assessments are colored by the political tendencies of those providing the judgments.  In fact, Coolidge presided over one of the greatest periods of prosperity in the history of our nation, and almost all of his actions -- and in many instances, wise inactions -- helped to steer the economy and social fabric of the country in a favorable direction.  Wilson, on the other hand, led us into a gruesome world war that served only as an appetizer for a more horrific reprise, shepherded in the 16^th and 17^th constitutional amendments that haunt us to this day, incarcerated innocent Americans for disloyalty, and bequeathed to us the Federal Reserve.

Surely, surveys that rank Wilson high and Coolidge low are biased to the left.  Examples of such tainted surveys include the Schlesinger polls (both père [1948, 1962] and fils [1996]) and the five Sienna College polls (between 1982 and 2010 -- the last of which absurdly ranked Obama the 15^th-best president).  Somewhat less biased are two polls done by the Wall Street Journal in collaboration with the Federalist Society (2000, 2005).

Of course, any definitive ranking will be subjective, but for my money, our least distinguished presidents prior to 1900 were Tyler, Fillmore, Buchanan, Johnson, and Arthur.

• Tyler.  Not so much obscure as unaccomplished, Tyler resigned both House and Senate seats and served an abbreviated term as Virginia Governor.  As president, he was referred to as "His Accidency," unable to decide whether he was a Whig or Democratic-Republican, humiliated by the resignation of his entire cabinet, and expelled from his party.  He was the first president to be the subject of an impeachment resolution.
• Fillmore.  His political career was largely confined to New York State, where he was an unsuccessful candidate for governor.  A surprise choice for VP, he favored slavery in the territories annexed from Mexico; attempted to appease both sides in the slavery debate, achieving no success on either side; maintained a confused position on the Compromise of 1850; signed the Fugitive Slave Act; and failed to unite the Whig Party, with some considering him the proximate cause of its death.
• Buchanan.  According to many, one of the two absolute worst presidents.  He was a perennially ignored candidate; a doughface, i.e., a Northerner with Southern sympathies, who ultimately alienated both sides; and unable to prevent the secession of southern states, thereby setting the stage for the Civil War.
• Johnson.  The other in the duo of absolute worst presidents, Johnson was a colossal failure as the surprise successor to Lincoln.  As a Democrat in a Republican administration, he was completely inept at implementing Lincoln's reconstruction plans and unable to contain the Radical Republicans.  He was the first president to actually be impeached.
• Arthur.  Not a failure as much as arguably the most obscure president in our history.  Having had almost no prior political career, he was an accidental choice as VP and a monumental surprise to America as president.  His two biggest failings were a dismal record on supporting Jim Crow and his concealment of the fact that he was ill in office with Bright's disease, which probably rendered him lethargic, ineffective, and erratic.

But more important than exactly which men constitute any list of worst presidents is the identification of a critical feature that many of these less than successful chief executives had in common.  Before ascending to the presidency, they were relatively obscure, their ascendancy was unanticipated, and from an objective standpoint, they were not particularly well-qualified.  Furthermore, the overwhelming majority ascended to the presidency via the death of the incumbent.  In summary, I would say that those presidents who arrived in office from obscurity, marking an occasion unanticipated by the American people and with precious little in the way of relevant experience -- especially if they ascended from the VP position -- are disproportionately represented among the worst presidents.

Between Arthur and Carter there were four more instances of a VP assuming the presidency upon the death of the president: Roosevelt, Coolidge, Truman, and LBJ.  The first and last were certainly not obscure.  The second and third were to some extent, but neither proved to be a failed president (both were reelected, as was Roosevelt).  LBJ was an awful president (the legacy of the Great Society and the Vietnam War continue to corrupt America's soul), but he was certainly a well-known commodity with, in principle, excellent credentials.

That said, it is the case that (depending on exactly who is on your list), from sometime in the late nineteenth century until the latter part of the twentieth century, there was no individual who ascended to the presidency from (relative) obscurity, in an unanticipated fashion, without sufficient credentials.  Perhaps this was because America became a major world power, so the scrutiny of presidential candidates grew more intense.  Perhaps it was because of the growing mass media, rendering it much more difficult for a dark horse to emerge.  Perhaps it was because of increasing party control over the nomination process.

Whatever the reasons, in the last 35 years, the perfect storm for a terrible presidency has occurred twice -- sans the death of an incumbent to abet it¹.  What changed after 1960 that allowed the American people to twice usher an obscure, unqualified, inexperienced, and ultimately fatally flawed candidate all the way through the nomination process and straight to the White House?

Here is an abbreviated answer: the cultural revolution that swept over America, most intensely from 1963-1974.  Among other unhelpful effects, the cultural revolution engendered a loss of faith among the American people in the social, political, and economic principles that undergirded the fabric of our nation for nearly two hundred years.  In particular, we questioned previously accepted axioms about what America stood for, how it should be governed, and the nature of the leadership it required.  This left the door open to the selection of obscure and unqualified candidates who promoted radical change in the nature of the country.  Snake oil salesmen like Jimmy Carter and Barack Obama marched through that door.  Not surprisingly, their presidencies, as with their pantheon of inept predecessors, have proven disastrous.

Notes

¹ I do not mean to suggest that Carter and Obama were the only awful presidents in that time.  George W. Bush betrayed his conservative backers by exploding the size of the government and its debt as badly as any president in the twentieth century, save perhaps FDR and LBJ.  Unlike Carter and Obama, though, he was neither obscure nor unanticipated.