It's always helpful to be reminded that early polls may not be predictive and that opinion can change, as was the case when Truman won in 1948 and when Carter lost in 1980. But we should keep in mind that today's polls are better and more frequent than they were 63 years ago.
Gallup's last 1948 poll was taken between Oct. 15 and Oct. 25 and showed Thomas Dewey leading Truman by only 5 points. No contemporary pollster would quit eight days before the election after getting that result.
There are in fact major differences between Truman's standing in 1947-48 and Obama's standing today. Contrary to Truman's "do-nothing" characterization of the Republican 80th Congress, it in fact did a lot. It repealed wartime wage and price controls, cut taxes deeply and passed the Taft-Hartley Act, limiting the powers of labor unions.
None of those actions was reversed by the Democratic Congress elected with Truman in 1948. Many congressional Democrats in those days were anti-New Deal conservatives. Truman won many votes from Democrats still upset about the Civil War. Few such votes will be available to Obama or congressional Democrats in 2012.
In addition, Truman's victory was brought about by two "F factors" -- the farm vote and foreign policy -- the first of which scarcely exists today and the second of which seems unlikely to benefit Obama in the same way.
When the nation went to war in the 1940s, 1 in 4 Americans still lived on farms. The 1948 electorate still reflected that America. Voter turnout was actually lower than it was in 1940, and the vast postwar demographic changes were not reflected in elections until turnout surged in the contest between Dwight Eisenhower and Adlai Stevenson in 1952.
Truman promised to keep Depression-era farm subsidies in place and charged that Dewey and the Republicans would repeal them. That enabled him to run ahead of Franklin Roosevelt's 1944 showing in 13 states with large farm populations, from Indiana to Colorado and Minnesota to Oklahoma.
Without that swing in the farm vote, Truman would not have won. Dewey, waking up to find that he would not be president as he and almost everyone expected, spotted that immediately the morning after the election.
Today only 2 to 3 percent of Americans live on farms. Farm prices currently are running far ahead of subsidy prices. Obama is not going to be re-elected by the farm vote.