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Former Sen. George McGovern now blames Democratic Party infighting for his loss to Nixon in the 1972 presidential race.
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Former President Gerald Ford (right), a Republican, lost his re-election bid to Democrat Jimmy Carter after a protracted nomination fight.
With the Democratic presidential primaries coming to an end Tuesday, the top two Democrats in Congress are prodding undeclared superdelegates to declare their support for either Hillary Clinton or Barack Obama by the middle of next week.
House Speaker Nancy Pelosi and Senate Majority Leader Harry Reid have good reason to want to wrap up the nomination battle as soon as possible: Historically, nomination battles fought right up to the convention have all ended with defeat in November.
Here is NPR's look at such contests:
In 1972, with inflation rising and the war in Vietnam dragging on, Republican President Richard Nixon sought re-election. His challenger was Sen. George McGovern, the antiwar Democrats' choice as a challenger. But the Democratic establishment was supporting former Vice President Hubert Humphrey, who refused to bow out of the race until the convention.
McGovern now blames that infighting as part of his landslide loss to Nixon, saying, "We were so badly scarred up by that battle the last 30 days for the nomination, and then it was carried right onto the convention floor, so that what the nation saw was a party in disarray."
Four years later, in 1976, Republicans were in disarray. The Watergate scandal had forced Nixon to resign from the presidency and Gerald Ford had taken over. But Ronald Reagan challenged Ford's bid for four more years in the White House — all the way to the party's convention in Kansas City.
Even as he conceded, Reagan spoke there of future generations as if he were the GOP nominee, saying, "Whether they have the freedoms that we have known up until now will depend on what we do here, Mr. President."
According to Stuart Spencer, who over his career advised both Ford and Reagan, Republicans were simply not convinced that Ford was their man.
"Ford had the machinery. He had the incumbency," Spencer says. "He had the power of the incumbency, and that is worth a lot of votes in a primary operation. And Reagan had sort of the hearts and souls of the Republican Party."
Ford lost to Democrat Jimmy Carter.
But in 1980, when Carter sought his party's nomination for a second term, Massachusetts Sen. Edward "Ted" Kennedy fought him all the way to the 1980 convention in New York, where Carter made this appeal: "I reach out to you tonight, and I reach out to all those who supported you and your valiant and passionate campaign, Ted. Your party needs, and I need you."
Only when Kennedy failed to change the convention rules in his favor did he finally concede — sort of.
"For me, a few hours ago, this campaign came to an end," Kennedy told the convention crowd. "For all those whose cares have been our concern, the work goes on. The cause endures. The hope still lives, and the dream shall never die."
Then, before the convention and the world, Kennedy snubbed Carter. CBS' Walter Cronkite described the bitter scene, saying, "Sen. Kennedy leaves the stand, sober, unsmiling. There will be no pictures in tomorrow morning's paper, and none for posterity, of Ted Kennedy holding Jimmy Carter's hand aloft."
Carter lost his re-election bid to Ronald Reagan.
In 1984, Democrat Walter Mondale, who had served as Carter's vice president, tried to keep Reagan from serving a second term. "I know the American people," Mondale said in a campaign advertisement at the time. "I am ready. I am ready to be president of the United States."
Meanwhile, Democrats had created superdelegates: party insiders and elected officials who could vote for the candidate they thought most electable. Another candidate, Democratic Sen. Gary Hart of Colorado, argued he was the most electable. Hart says that's why he kept running right up to the 1984 convention.
"Because I had won 11 of the last 12 primaries, including a sweep of California, and Vice President Mondale did not have a majority," Hart said recently. "The process continued on, and it was a struggle for whether I could convince any superdelegates to change."
He could not, though. Mondale went on to win the nomination — and lose the general election to Reagan.
Colby College elections expert Sandy Maisel says Hart pinned too many hopes on superdelegates.
"With all due respect to Sen. Hart, I think in 1984 most of the superdelegates said they would be much more comfortable with Walter Mondale in the White House than they would have been with Gary Hart in the White House," Maisel says.
Since 1984, there have been no more battles fought all the way to party conventions — at least not until now.
When asked Sunday on CBS' Face the Nation whether Hillary Clinton was willing to take her fight to the Democrats' Denver convention in late August, Clinton adviser Howard Wolfson did not rule it out.
"The goal and the hope and the expectation is that we will have a nominee well before then," Wolfson said. "We still believe that that nominee can and should be Sen. Clinton."
And just as underdog Hart attempted 24 years ago, the Clinton campaign still hopes to persuade superdelegates that she would be the stronger Democratic nominee.