Some history and when does it end?

Ken Rudin of NPR writes in Political Junkie prior to yesterday's results:

Sometimes, you need to decide for yourself when it's time to pull up stakes and fold your cards. There were two vastly different approaches in 1980. George H.W. Bush, in his bid for the Republican nomination that year, went into the May 20 primary badly trailing front-runner Ronald Reagan. Bush's victory in Michigan was substantial — 57 percent to 32 percent. But Michigan still wasn't enough. Less than a week later, he figured there was no way he could surpass Reagan in the delegate count and ended his campaign.

On the Democratic side that year, it wasn't as tidy. Sen. Edward Kennedy (D-MA) took his fight to President Carter behind the last round of primaries in early June — when Carter won a majority of the delegates needed to assure him renomination — all the way to the convention, where the Kennedy forces fought the Carter forces over the rules and platform, all unsuccessfully.

It has been said that when a battle for the nomination goes all the way to the convention — as with Kennedy vs. Carter in 1980, and Republicans Reagan vs. Ford in '76 — the party's task of uniting in time for November is that much more difficult. This may be true, though the defeats of Presidents Ford and then Carter are more complicated than that.