On Slate, David Greenberg argues that this primary is not a historical anomaly, that democratic candidates are typically chosen late in the game. He also claims that the failures of deomcratic candidates in 1972, 1980, and 1984 had nothing to do with the long selection process. He says they would've failed anyway because they (McGovern, Carter, Mondale) were weak candidates. Tsongas, too, in 1988, though Greenberg concedes that the good governor was indeed hurt some by the primary campaign.
My only point is this: No one really can know what would have happened had those nomination processes been shorter. But we know here and now that a drawn out primary fight will get nasty, it could divide the party, and it could certainly give Republicans a greater opportunity and more ammunition with which to hold on to the White House than they would otherwise have had. (I refer to the red phone ad and Clinton's statement that she and McCain are the candidates of experience.)
All this to say, history is not only cold comfort indeed but an argument for Clinton to withdraw.