For Democrats, the 2008 primary season has been the longest in recent memory, with no shortage of upsets, talk of momentum, and states defying parties to hold primaries earlier than ever. Three historians discuss the significance of 2008's lengthy primaries.

After trading wins in Democratic primary contests, the race continues between Sens. Hillary Clinton and Barack Obama for the party's nomination, while GOP Sen. John McCain works to mount a fresh general election strategy. A panel of experts examines the…