Let's suppose that, one fine Agatha Christie morning, Republican Johnny Isakson and Democrat Jim Barksdale escort Libertarian Allen Buckley to the very edge of those tall, white cliffs of Dover.

Around lunch time, the Democrat and Republican return, famished and all by themselves. In the ensuing investigation, Miss Marple would be hard put to say which candidate most wanted to see Mr. Buckley disappear into the abyss.

On Nov. 9, the Libertarian could be responsible for pushing Georgia’s race for U.S. Senate into the state’s first nine-week, general election runoff, forcing two candidates to campaign through Thanksgiving, the SEC football championship, Hanukkah, Christmas, Kwanzaa, New Year’s Day, and the 2017 Golden Globe Awards.

The prospect cuts to the very root of Georgia’s love-hate relationship with the insurrections promised by third-party candidates.

This week, Secretary of State Brian Kemp announced that Green Party presidential candidate Jill Stein had fallen 1,575 petition signatures short of the 7,500 to have her name placed on the November ballot.

Many Georgia Democrats cried crocodile tears at the news. The plurality required to win the state’s 16 electoral votes is within Hillary Clinton’s reach. Even nominal, liberal competition could deprive Clinton of the percentage point she might need to gain a plurality victory over Donald Trump in Georgia.

My Journal-Constitution colleague Kyle Wingfield has even marveled that the usually nimble Georgia GOP neglected to make sure Stein didn’t get those extra signatures she needed.

Likewise, some Republicans now bemoan the presence of Libertarian Gary Johnson on the Georgia presidential ballot. Johnson has given anti-Trump but GOP voters a place to shelter – creating the underpinnings for a potential Clinton victory here.

Georgia Democrats, needless to say, are very happy to have Johnson on the ballot.

But such bipartisan vacillation over third-party candidates stops when you drop to Georgia’s race for the U.S. Senate. The hostility is mutual, shared by both Republicans and Democrats. But it is entirely unspoken.

“The strength of Jim Barksdale’s outsider message is resonating with Georgians and we will be well positioned to win on Nov. 8 or Jan. 10 in the event of a runoff,” insisted Barksdale campaign manager Dave Hoffman.

From Isakson himself: “We just keep moving the ball down the field every day, and elections have a way of rewarding the best team with the most votes.”

The political reality can be found in a Barksdale overture to Isakson, made earlier this month, for six one-on-one debates on the economy. Buckley, the Libertarian, was not included. Initial talks between the Barksdale and Isakson campaigns could come as early as this week. (At least two three-candidate debates are in the offing this fall, though not certain.)

The Democratic and Republican preference for a two-party race requires a brief history lesson.

The 1990s gave Georgia two exceedingly tight U.S. Senate races that still resonate. Libertarians were involved in each.

In 1992, Democratic incumbent Wyche Fowler finished with 49.23 percent of the November vote, less than the state-mandated majority. Three weeks later, in the resulting runoff, Republican Paul Coverdell took the seat with 50.6 percent of the vote.

Noting the role of the third-party candidate in that contest, the 1994 state Legislature, then in Democratic hands, decided that, in the future, a plurality (above 45 percent) was good enough to win a general election.

The change worked. In 1996, Democrat Max Cleland won Sam Nunn’s open seat with 48.9 percent of the November vote. There was no runoff. The Libertarian had been negated.

Then came the GOP wave in Georgia. In 2005, as Gov. Sonny Perdue geared up for re-election, Republicans realized that runoffs were their friends.

Older voters in Georgia tend to be disproportionately white. They are also the most reliable voters in low-turnout runoffs. So the majority rule was restored to November elections.

As it turned out, Perdue had no need of it in 2006. But obeying the law of unintended consequences, the renewed standard tripped up Republican incumbent Saxby Chambliss (he had defeated Cleland six years earlier) in the 2008 U.S. Senate race.

Chambliss’ 49.8 percent performance pushed him into a five-week runoff with Democrat Jim Martin. On the first Tuesday after Thanksgiving, Chambliss won the runoff with 57.4 percent of the vote.

Fast-forward to 2013. The U.S. Justice Department insisted that Georgia extend its runoff periods, in both primaries and the general election, to permit the casting of overseas ballots. Not three, not five, but nine-week follow-up campaigns became the agreed-upon standard.

(Many Republicans think the state was being pressured to abandon runoffs altogether, in favor of plurality elections.)

The 2014 Republican primary runoff, pitting businessman David Perdue against then-congressman Jack Kingston, was the first subjected to the nine-week gauntlet, extending the race from May to July.

“You’re finishing off a long process. It should be a smaller, tighter race,” said Kingston, currently a Trump surrogate. “Politics does have somewhat of a season, almost like a sport.”

Kingston, of course, lost the primary runoff. But the general election that followed, featuring Perdue and Democrat Michelle Nunn, again raised the prospect of a nine-week extension.

“I thought nine weeks were better than four weeks,” said Jeff DiSantis, the Nunn campaign manager. “Significantly better, but still not great.” Voting habits of a reduced electorate would still greatly favor the Republican side.

Perdue’s eight-point victory ended the November 2014 contest.

Historically, Isakson has done better. In the last 24 years, only three U.S. Senate contests in Georgia have been won with double-digit margins. Isakson is responsible for two of those.

But many Republicans, here and in Washington, worry that the less-than-stellar performance of the Trump campaign in Georgia could push Isakson under 50 percent on Nov. 8.

If that happens, chances are that the outcome of the nine-week extension, after the spending of several millions of extra dollars and leaping over several holidays, would follow the pattern established in 1992.

However, Mr. Buckley, who was not pushed off a cliff on that Agatha Christie day, likes to think that he could break the mold and make it into a runoff with Isakson. “The odds are against it, but the odds are much better than in the past,” he said.