Which past presidential election offers this year's closest parallel?
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Every four years since 2000, during the fall semester, I've taught a college course on that year's unfolding presidential election. In the past, it's been fairly easy to point to a prior election year that offers illuminating historical parallels. But this time, I'm finding that the problem is not in identifying a parallel year; it's that the best comparison election keeps changing. Below are the main contenders.

The 2000 election: Last summer, it looked like 2000 would offer an almost perfect analogy. That year, Vice President Al GoreAlbert (Al) Arnold GoreJoe Lieberman: We’re well beyond partisanship, our national government has lost civility Trump doesn't start a trade war, just fires a warning shot across the bow Dems face hard choice for State of the Union response MORE represented continuity with the incumbent administration and faced mostly token opposition in the Democratic primaries from a respected liberal sitting senator, Bill Bradley (N.J.). The Republican side was more heavily contested, but it seemed most likely that a son of former President George H.W. Bush would prevail.

However, this relatively placid 2000 parallel has long since fallen apart: Sen. Bernie SandersBernard (Bernie) SandersDems ponder gender politics of 2020 nominee 2020 Dem contenders travel to key primary states After Florida school shooting, vows for change but no clear path forward MORE (I-Vt.) is proving much more formidable than Bradley, former Florida Gov. Jeb Bush is already out of the running for the Republican nomination, and many other factors have come into play.

The 1988 election: So, then, can 2016 be viewed as parallel to 1988? As in 2000 and 2016, in 1988 the party in power was fighting for a third consecutive term in the White House. Incumbent Vice President George H.W. Bush contended, and ultimately prevailed, among several Republican rivals. He did so in the primaries without the explicit endorsement of sitting President Reagan, who had defeated Bush for the GOP nomination eight years earlier. Today, quasi-incumbent Hillary ClintonHillary Diane Rodham ClintonWoman behind pro-Trump Facebook page denies being influenced by Russians Trump: CNN, MSNBC 'got scammed' into covering Russian-organized rally Pennsylvania Democrats set to win big with new district map MORE, the former secretary of State, has narrowed the original Democratic field from five to two and may soon dispatch Sanders as well — all without having received a primary-season endorsement from her rival-turned-boss, President Obama.

Meanwhile in 1988, a gaggle of often-unimpressive Democratic contenders came to be called the "Seven Dwarfs," prefiguring the even more scattered original 17 GOP candidates for this year's contest. But this year, Republican front-runner Donald TrumpDonald John TrumpAccuser says Trump should be afraid of the truth Woman behind pro-Trump Facebook page denies being influenced by Russians Shulkin says he has White House approval to root out 'subversion' at VA MORE has certainly not turned out to be an electoral dwarf. And so the parallels to 1988 have faded.

The 1948 election: Most recently, 2016 has begun to take on shades of 1948, a year that stands out for showcasing four distinct points on the political spectrum. Two of these represented minor-party bids: At the far left, former Vice President Henry Wallace staked out an explicitly socialist perspective, while on the far right, arch-segregationist Sen. Strom Thurmond of South Carolina pandered to, and stoked, overt racial animosity. On the center-left, incumbent President Truman took a liberal posture at home and an aggressive stance abroad, while the center-right GOP nominee Gov. Thomas Dewey (N.Y.) represented old-fashioned, "small-c" conservatism.

The chaos of the 1948 election went right down to the wire, indelibly memorialized by the Chicago Tribune's premature headline "Dewey Defeats Truman." In 2016, if Sanders, Trump or some other credible figure were to press forward with serious minor-party bids, this year's electoral disarray might still come to resemble that of 1948. But, thus far at least, all of this year's electoral action has remained within the two major parties. And so 1948 may also not end up being the best analogy for 2016.

The 1968 election: Still another — quite depressing — parallel seems possible: that 2016 will evolve to resemble the excruciating spectacle of 1968. In this scenario, Trump mostly closely resembles former Alabama Gov. George Wallace, who fused economic populism and undisguised racism into a potent appeal to poorer and less-educated whites. Notably, however, Wallace did so as a third-party candidate, not as the standard-bearer of a major party, as Trump threatens to do.

Oddly enough, Hillary Clinton offers significant similarities to both of the other major 1968 contenders. Like the Democratic nominee then, Vice President Hubert Humphrey, Clinton presents an almost wearily familiar face seeking a third-term for the party in power. And she does so at a time when an unsettled electorate seems unhappy with all of its presidential choices.

But the electoral figure that Clinton has come to most resemble is Richard Nixon. In 2016, Clinton is widely loathed by opponents and largely unloved even within her own party, although begrudgingly respected for her depth of experience. She is extraordinarily well-qualified, yet can be politically heavy-handed and personally awkward. But, also — much like Nixon — Clinton has been on both the winning and the losing side of several prior presidential elections, and knows how to grind onward to electoral victories.

In 2016, Democrats' best hope may well be that Clinton is able to get into the Oval Office using a slow-but-steady, "I am not a crook" Nixon-style campaign. Of course, under the shadow of Benghazi, classified emails and other as-yet-undetected threats, Democrats will also need to hope that Clinton doesn't end up departing the Oval Office in the same way that Nixon did. That would definitely be one parallel too many.

Smith, Ph.D., a senior fellow with the Progressive Policy Institute, teaches political science at Columbia University and New York University.