It was 1976. Gerald Ford, the sitting president, had won 16 of 27 Republican primaries and led in the party's delegate count. But Ronald Reagan carried the nomination battle into the convention anyway.

Why won't some candidates concede?

Hillary Clinton's chances of winning the Democratic nomination are increasingly remote, with even a blow-out victory in Tuesday's West Virginia primary unlikely to make much difference. Still, the New York senator has vowed to continue campaigning until the nominating process ends June 3 in Montana, South Dakota and Puerto Rico.

Political analysts say office seekers who hang on, even long after the race seems futile, may be hoping to position themselves for a later run or to reshape the party more to their liking. They may be bargaining for the vice presidency, feel pressure from supporters or believe there is an off-chance they will get lucky.

But what is clear from 1976 and two more-recent races is that the party took a drubbing when challengers refused to concede and instead pursued the nomination into the convention. "The lesson is that you can make matters much worse for your party and yourself if you push this too far," says American Enterprise Institute scholar Norm Ornstein.