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The U.S. Senate Debate for Louisiana gets underway at Georges Auditorium including David Duke, a convicted felon and Ku Klux Klan grand wizard, attorney Caroline Fayard, and U.S. Rep. John Fleming at Dillard University in New Orleans, La. Wednesday, Nov. 2, 2016.

Invite a guy who's desperate for attention onto a crowded debate stage, and of course he's going to suck up all the oxygen.

And so Wednesday night's second and final U.S. Senate primary debate devolved into a debacle centered on the one participant who has no shot of becoming Louisiana's next United States senator, David Duke.

Including the former Ku Klux Klan Grand Wizard and convicted felon was sponsor Raycom Media's decision, based solely on the findings of its own outlying poll, the only recent survey that has shown him topping — just barely — the 5 percent threshold.

The other candidates, state Treasurer John Kennedy, Public Service Commissioner Foster Campbell, lawyer Caroline Fayard and U.S. Reps. Charles Boustany and John Fleming, each scored at least twice that high, and are all running well-funded, professionally managed campaigns. Duke's effort so far has amounted to little more than an invective-laden Twitter feed and a couple of robocalls aimed at linking himself to the man he says inspired his run, GOP presidential nominee Donald Trump.

Yet despite moderator John Snell's repeated pleas that the debate not become a referendum on one candidate, that's just what happened.

Duke's opponents let their distaste show, which was appropriate and which would have been fine had it stopped there. It didn't. Instead, Duke engaged the longtime WVUE anchor in a lengthy shouting match over his ridiculous contention that he was railroaded by the feds, despite his admission under oath that he'd defrauded his own loyal contributors and not paid taxes on his ill-gotten gains.

And when Snell confronted Duke on the virulent bigotry he's spewed throughout the campaign by calling him on his reference to "the CNN Jews," Duke responded by insisting "there is a problem with a very strong powerful tribal group" that dominates media and banking.

"I am not opposed to all Jews. I think there are a lot of great Jews. I think there are a lot of Jews I honor, but let me tell you something — I am against Jews, or anybody else, that puts interests of some other place, another country, over our own country," he said. That exchange was highlighted by national journalists watching with horror from afar, as was Duke's assertion that Hillary Clinton belongs in an electric chair. Good times.

There was a second, parallel conversation among the non-fringe candidates, and it largely echoed their interactions to date.

Campbell and Fayard once again tangled over who's the better Democrat, but this time, they did it in part by using Duke as a foil. Fayard interrupted her own answer to a question on the Affordable Care Act to renounce Duke as a slithering snake, and defended her controversial ad that linked Campbell and Duke based solely on an awkward photographed handshake and an out-of-context quotation from a prior debate. Campbell angrily rejected any suggestion of affinity and positioned himself as a long-standing champion of the little guy with a record to prove it.

The Republicans, meanwhile, jockeyed for relative position on the right. The back-and-forth between Fleming and Kennedy was particularly pointed, with Fleming casting Kennedy as a closet liberal and irresponsible steward of his own state office, and Kennedy reciting a list of other Republicans who had once been Democrats— Trump included — and dismissing Fleming as a Washington insider. Boustany, who has tried to position himself as the wonky grown-up in the race, seemed to be dialing in from another, more substantive debate.

On issues, though, the candidates mostly fell along party lines on topics ranging from health care to their standards for supporting U.S. Supreme Court nominees, with only minor variations among members of each party. Had circumstances been different, Snell, a skilled and experienced moderator, might have been able to get a good conversation or two going. With Duke front and center and debate organizers visibly flustered, that just wasn't in the cards.

Instead, the real candidates got short shrift. So did historically black Dillard University, which hosted the debate and had to deal with student frustration over Duke's presence and the organizers' refusal to allow them into the empty auditorium where the forum was staged. And so, more than anyone, did any viewers who are still weighing which of the non-fringe candidates deserves their vote.

But hey, David Duke got his moment in the spotlight. Can we all at least agree that it should be his last?

Follow Stephanie Grace on Twitter, @stephgracela.