It was the eve of the 1980 Republican convention in Detroit, and presumptive nominee Ronald Reagan was dumping all over his defeated rival, George Bush. Reagan kept mentioning "voodoo" economics, Bush's signature line during the primaries to describe Reagan's fiscal policies.

"He was going off on him pretty good," recalls GOP consultant Stuart Spencer, a close adviser to Reagan and the recipient of his rant against Bush.

Three days later, Reagan picked Bush to be his running mate. This came as no surprise to Spencer. "Reagan was a pragmatist," he says. The conservative Reagan needed Bush's moderation to balance the ticket. "Voodoo economics" might have stung, but it did not cross "The Line."

In the end, it's all about The Line. Did the rival candidates for president cross the line of decorum during their primary campaigns far enough to preclude a partnership during the general election, and perhaps beyond?

They often do, which is a big reason why presidential nominees are reluctant to pick former opponents to be their running mates, and why Democrat John Kerry's selection of former rival John Edwards is such an anomaly.

And why the political class was abuzz yesterday with talk of diapers.

"When I came back from Vietnam in 1969, I'm not sure John Edwards was out of diapers then yet or not," Kerry said just before the Iowa caucuses in January. This is not an image that conveys seasoning and gravitas, which is exactly what Kerry had in mind about the young-looking Edwards, a first-term senator and then 50.

Never mind that Kerry seemed to regret the remark and immediately tried to backtrack. "I truly don't want to be negative about anybody," he said later in the presentation. "That comment I made was not meant to be negative. I don't want to go that road."

Alas, he did, and the put-down likely will serve as video-ready fodder in all manner of offensives from the Bush-Cheney campaign.

But did it cross The Line?

"The diaper remark? No," says William Cohen, a former Republican senator from Maine who served as Bill Clinton's defense secretary.

Not even close compared to the level of invective that marked the race between George W. Bush and John McCain four years ago. Or when challenger Bob Dole fired at eventual nominee George H.W. Bush, "Stop lying about my record," on the night of the New Hampshire primary in 1988.

"That probably sealed it for Dole," says George Edwards, a professor of political science and the director of the Center of Presidential Studies at Texas A&M University. Bush picked Dan Quayle.

"When you're campaigning against someone in a primary, chances are you're going to say things that will make you hate each other more than the nominee of the other party," says former Nebraska senator Bob Kerrey, who sought the Democratic nomination in 1992. Eventual nominee Clinton considered him as a possible running mate before he eventually picked Al Gore.

Former rivals can form winning tickets. GOP nominee Herbert Hoover and defeated challenger Charles Curtis teamed up to win the White House in 1928. Franklin Delano Roosevelt picked his former opponent, John Nance Garner of Texas, who served as FDR's vice president for two terms. Reagan picked Bush, which worked out well in the general election -- Reagan beat Jimmy Carter in a landslide.

It also worked when John Kennedy picked former rival Lyndon Johnson. But this partnership required both men to swallow hard and overlook several breaches of The Line. As late as the 1960 Democratic convention in Los Angeles, Johnson's campaign was spreading word that Kennedy suffered from Addison's disease, an incurable disorder of the adrenal glands that JFK was diagnosed with in 1947. This was after the Kennedy campaign had launched a whisper campaign that Johnson was not healthy enough to be president because he had suffered a heart attack in 1955. The two senators were regional, temperamental and philosophical opposites, acutely ill at ease with each other.

"It was an oil-and-water ticket, but they needed Texas to win the election so that was that," says political scientist Steven Schier of Carleton College in Minnesota.

One might assume that unsuccessful presidential primary candidates -- pre-vetted, experienced campaigners of self-selecting drive -- would make a natural pool of Number 2's. Yet they do not.

In the first half of the 20th century, this was generally a case of candidates simply not wanting to be vice president, says William Mayer, a professor of political science at Northeastern University. Mayer says the vice presidency was considered a "zero kind of office." Until Richard Nixon, the job was not seen as a pathway to winning the presidency. "The only way it was a steppingstone was if the president died," Mayer says.

That changed when the job of vice president changed and contemporary deputies such as Walter Mondale, Al Gore and Dick Cheney played key roles in their respective administrations. Running mate became a more coveted job. But human nature doesn't change. People get annoyed, they nurse grudges and prospective "dream tickets" -- say, George W. Bush and John McCain -- are never realized.

The precedents are encouraging for Kerry and Edwards, Mayer says. The last losing ticket made up of former rivals was Adlai Stevenson and Estes Kefauver. That was in 1956. Edwards was 2 years old and barely out of diapers.

During the Democratic presidential primaries, John Kerry, left, questioned John Edwards's qualifications. Yesterday he chose Edwards as his running mate.John Kennedy, right, talks with running mate Lyndon Johnson, a former rival for the Democratic nomination, in July 1960.