A recent opinion survey asked Americans to rate post-World War II presidents. Reagan was named the best, followed by Clinton and Kennedy and Obama was tagged as the worst, followed by George W. Bush and Carter. With the exception of Kennedy, presidencies that preceded Carter were almost invisible in the balloting.

That's probably because of the comparative youth of most of the respondents. People who remember the building of the Berlin wall and cast their first votes for Nixon are now in their 60s and 70s; the most significant political event for first-time voters in 2012, such as my students, was 9/11. They see Truman, Eisenhower, Kennedy, Johnson, Nixon and Ford as figures to be read about in history books, along with Wilson and Coolidge. I believe the only reason Kennedy's name breaks through the chronological memory barrier is because his assassination gave him a unique identity that has endured.

I'm on the old side of that barrier. My first vote went to Eisenhower, the first president I met personally was Kennedy, the one in whose administration I served was Nixon and the one whose campaign staff I was asked to join was Reagan. I remember the earlier presidencies and believe they can give us a useful perspective on the relevance of the current poll.

Truman still holds the record for the lowest approval ratings ever recorded. When he launched what was perhaps the most important initiative of his presidency, the reconstruction of Europe after the war, it was called the Marshall Plan, after his Secretary of State, because it would not have passed the Congress if it had been called the Truman Plan. Adlai Stevenson, the Democratic nominee while Truman was still president, campaigned against " the mess in Washington." His contemporary Americans would have put him high on the "worst" list but Truman is now considered one of the best postwar presidents.

Eisenhower was dismissed by academics as a "passive" president, one who did nothing to change things. However, my Senate colleague Pat Moynahan (D-NY), said that Eisenhower changed America more than the two "activist" Presidents the academics liked — Kennedy and Johnson — did by building the Interstate Highway System. Like Truman's, Eisenhower's standing among historians has been rising steadily through the decades.

Kennedy's approval rating soared after his assassination. However, we now know that he lied about the seriousness of his illness and was predatory in his relationships with women. He was also the president that sent troops in force into Vietnam Nam, turning an advisory mission into a war that eventually cost more than 10 times as many casualties as Iraq. His apologists say he would have pulled out of Vietnam if he had been re-elected, but if he had realized that he had made a ghastly mistake, why would he let a potential election delay an action that would have saved tens of thousands of lives? Historians are nowhere near as high on him as his contemporaries were.

As for Johnson and Nixon, both won huge landslides but both left office as hated men.

The point is that polls are snapshots in time, subject to the uncertainties of temporary public opinion, not definitive assessments of actual accomplishment. That doesn't mean that they don't matter — Truman, Johnson, Nixon, Carter and George W. Bush could all explain to Obama how difficult his final two years will be because his approval ratings are in the tank — but no one should take last week's ratings as history's final judgment.

Robert Bennett, former U.S. senator from Utah, is a part-time teacher, researcher and lecturer at the University of Utah’s Hinckley Institute of Politics.