That is because, with rare exceptions, Democratic primary and caucus voters reject the candidate who leads in the polls and nominate, instead, some semi-unknown underdog.
Let's look at the record. Only twice in the last 44 years has the Democratic nominee for president emerged in the year before the election as the clear front-runner in the Gallup Poll: Former Vice President Walter Mondale in 1984 and then-Vice President Al Gore in 2000.
By contrast, every other eventual Democratic presidential candidate since (with the obvious exception of uncontested incumbent President Bill Clinton in 1996) has trailed -- often badly -- in surveys the year before the election.
Consider the polling record of the only Democrat since FDR to win two White House terms -- Bill Clinton. In August 1991, the Arkansas governor was running fifth with 11 percent, badly trailing New York Gov. Mario Cuomo, the Rev. Jesse Jackson, U.S. Sen. Lloyd Bentsen of Texas and former California Gov. Jerry Brown.
By that October, just 13 months before he would defeat President George Herbert Walker Bush, Clinton had slipped to a discouraging 6 percent in the Gallup.
Yes, John F. Kennedy was running second to two-time nominee Adlai Stevenson in polls conducted in January, April, May and November of 1959. But JFK looked like a world-beater compared to other eventual Democratic standard-bearers. Vice-President Hubert Humphrey, the 1968 nominee, was the first choice of just 6 percent of Democrats in September 1967.
Four years later, Sen. George McGovern went from 5 percent support in January all the way to 5 percent support in December, when he still trailed both senators Edmund Muskie and Ted Kennedy by more than 20 points.
Former Georgia Gov. Jimmy Carter, who won the presidency in 1976, did not register -- not even in single digits -- in any of the four 1975 polls. By August 1979, President Carter was again the underdog, trailing Sen. Kennedy by 63 percent to 25 percent before the Iranian hostage crisis rescued his political career.