President Jimmy Carter and Senator Edward M. Kennedy had been sharp adversaries with a bad history, and in the 1980 presidential campaign they let it bleed into a bitter nomination fight. The Carter administration challenged Mr. Kennedy’s patriotism and refused to debate, while Mr. Kennedy dragged out their fight for nine months, all the way to the Democratic convention. A weakened Mr. Carter prevailed and won the nomination, but he went on to lose in November.

Convention fights often spell ruin for a party. The 1980 experience for Democrats — as well as a fight in 1968, and one in 1976 for Republicans — all suggest that a bruising primary carried through the summer can contribute to defeat in November.

Today, nervous Democrats are worried that history will repeat itself as Senator Hillary Rodham Clinton, who lags in delegates and the popular vote, has refused to concede the nomination to Senator Barack Obama. Despite the increasing rancor of the campaign, Mrs. Clinton says she is staying in until the voting is over.