When it comes to the last half century, the second-term curse might seem real. For several of the seven modern second-term presidents, a single image of failure overshadowed many of their achievements:
•Clinton, before impeachment proceedings, looks straight into the TV cameras to utter the most famous line of his presidency: "I did not have sexual relations with that woman."
•Nixon, hands held high, fingers spread in a V-for-victory sign, boards a helicopter on the White House lawn to leave the White House, the first and only president to have resigned in office.
[Editor's note: The original version of this story wrongly said Nixon was impeached. He resigned in the face of almost certain impeachment.]
•Johnson, eligible to run for president again but so unpopular he is about to lose the Wisconsin primary to little-known Minnesota Sen. Eugene McCarthy, announces on TV, "I shall not seek, and I will not accept, the nomination of my party for another term as your president."
Even generally successful presidents have faced humiliating episodes: for Reagan, the Iran-contra scandal; for Dwight Eisenhower, the U-2 spy plane episode. But there's a difference between difficulty and debacle.
"Generalizations are tricky," says Mr. Greenberg, whose 2003 book, "Nixon's Shadow: The History of an Image," examined one second-termer. "When we start looking at the evidence, there's not a lot to substantiate the notion."
How can you distinguish between the problems presidents encounter in every term and ones that discredit the entire four years? What's the standard for gauging any president's success?
Even those who have tried to develop measures are cautious about generalizing. In their book, "Addressing the State of the Union," University of Northern Iowa professor Donna Hoffman and Dominican University of California assistant professor Alison Howard use one such tool: calculating how many legislative requests presidents make of Congress – and how many get adopted in the next session.