Katharine Seelye makes a good point that in non-incumbent years, Democrats often do not nominate the frontrunner ...
Starting in 1952, the Democrats have contested the presidency 11 times (not counting 1964, 1980 and 1996 when they nominated incumbents). Of those 11 times, only five of the candidates who were leading in national polls in January won the nomination: Adlai E. Stevenson in 1956, John F. Kennedy in 1960, Walter F. Mondale in 1984, Bill Clinton in 1992 and Al Gore in 2000. The so-called front-runners who did not win the nomination included Estes Kefauver in 1952 (Stevenson won the nomination), Lyndon B. Johnson, who shared the lead with Robert F. Kennedy in 1968 (Hubert H. Humphrey was nominated after Johnson dropped out and Kennedy was assassinated), Edmund S. Muskie in 1972 (overtaken by George S. McGovern), Edward M. Kennedy in 1976 (Jimmy Carter pulled ahead), Jesse Jackson and Gary Hart in 1988 (Michael J. Dukakis took over) and Howard Dean in 2004 (lost to John Kerry).
Of course, this is in stark contrast to Republicans who, as it's been said, like to run elections the way a Rotary Club does (with the nominee decided years in advance) ...
So should Hillary be worried?