THE MISTAKEN FAITH IN ENVIRONMENTALISM:

Re-Assessing Obama’s “Under-Performance” in the Polls (Jeff Jones, 8/19/08, Gallup)

There have been only five non-incumbent presidential elections in the modern polling era. I'm largely evaluating the political environment for each using the outgoing president's approval rating, since Gallup has data for that going all the way back to the 1940s, and we know approval is correlated with other political environment ratings such as satisfaction and ratings of the economy.

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In 1952, Democratic incumbent Harry Truman had historically low approval ratings and Republican Dwight Eisenhower easily defeated Democrat Adlai Stevenson for president. The political environment explanation would predict a solid Republican win, and that is what happened.
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In 1960, Eisenhower was very popular when he left office after eight years (58% approval in the last measurement before the election), but John F. Kennedy and the Democrats won a closely contested election (winning the popular vote by less than a percentage point) to replace Eisenhower instead of choosing his vice president, Richard Nixon. The political climate model does not seem to apply to this election.
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In 1968, the Democrats were trying to win an election with an unpopular president (Lyndon Johnson) waging an unpopular war (in addition to other problems going on in the country), but Republicans only narrowly won. A political conditions interpretation would suggest a big Republican win in 1968, not a 1-point win. It's possible that George Wallace's strong third-party showing made things closer than they otherwise would have been.
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In 1988, Ronald Reagan was a popular incumbent (51% approval), though he was not as popular as Eisenhower in 1960 or Bill Clinton in 2000. The elder George Bush won what many perceived as a third Reagan term, and by a healthy margin.
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In 2000, Clinton had high approval ratings (57% at the time of the election) and national satisfaction was near record levels, yet Al Gore won the popular vote by only the narrowest of margins.

Whereas a political climate explanation seems to work so well in incumbent presidential elections and midterm elections, on the surface it doesn't seem to explain the outcomes of non-incumbent elections that well, in terms of either the margin or the winner.